He has been up "from the early hours", his working day now devoted to juggling the demands of family, rock stardom and the ongoing campaign for African aid and debt relief. U2's long-awaited new album, No Line on the Horizon, is finally finished. "It began and ended in a flash," he says. "The last 24 hours were just extraordinary. It was like Chinese calligraphy, where the monks take ages to mix the ink and then - bam! - it all happens in seconds."

In three days' time, the band will fly to Washington, where they will perform Pride, their Martin Luther King song, and City of Blinding Lights, their Barack Obama song, on the steps of the Lincoln Memorial. "The world is waking up again," says Bono. "It's going to be a tough transformation, but it's going to be exciting. Things are shifting in surprising ways."

Over the next hour, Bono will talk about what it means to be the world's biggest rock star and the world's most famous global campaigner, about music and faith and activism, and the tensions his high-profile tightrope walk has caused in the band. He will also talk about U2's new music, and the shift in his song-writing style away from the first person ("I'd just worn myself out as a subject matter").

No Line on the Horizon is U2's 12th studio album. It sees the world's biggest band challenging themselves - and their audience's expectations - in a way that they have not done since the 90s' experimentation of Achtung Baby and Zooropa. It was, though, a difficult and protracted birth, and I was a witness to its gestation. In the original plan, hatched almost two years ago in a casual conversation with Bono, I had been invited to Fez to track the making of the new album, stage by stage, from inception to completion. So it was that, what seems like an eternity ago, I boarded a plane to Morocco.

Fez, Morocco, June 2007

Bono, guitarist The Edge, drummer Larry Mullen and bassist Adam Clayton are gathered, with producers Brian Eno and Daniel Lanois in the ancient North African walled city to start recording a handful of new songs. Their "studio" is the enclosed courtyard of a riad on the edge of the medina. Moroccan carpets have been spread across the stone floor, ornate pillars and spreading palms tower over the amplifiers and sound desks, and, from time to time, small birds dart overhead, startled by the constant bursts of rough and ready music.

The mood matches the makeshift setting: a batch of new songs, tentative, half-formed, sketchy, are elaborated on or set aside for future reference. Eno, who has assumed the role of musical director, shouts out tempo changes, instructions, suggestions. "The chords sound a little too vanilla," he says of one laid-back, swampy groove. Bono, who has a couch all to himself, concurs. "We need to find that nightclub-in-Tripoli feel," he shouts back, swaying to the beat, "then move it on down to Bamako." The vibe is one of unhurried creativity, the six musicians - Eno on keyboards, Lanois on guitar and pedal steel - stretching out and enjoying themselves. It feels like the beginning of a new adventure.

"What's happening down here is beyond reason," Bono had enthused, when the idea of me shadowing them had first been broached. "Spirits are hovering. We're chasing the Joujouka drummers and different structures for pop."

The legendary Joujouka drummers drew both Brian Jones and William Burroughs to Fez in the late 60s, but this time around, other guiding spirits were also at work. Every night, as darkness fell, the haunting voices of devotional Sufi singers would rise up and drift across the rooftops, their song-prayers lasting for hours at a time. "There was definitely something in the air down there," Bono will tell me later. "And we picked up on it."

Could he describe what that something was exactly?

"Not without sounding pretentious," he says, laughing. "I mean, a lot of people have gone there, searching. There's a bit of the Mighty Boosh about it. Out in the desert, looking for the new sound. Have you seen that episode where they are out in the desert looking for the new sound? They find Chris De Burgh and he's been out looking for the new sound for 10 years [laughs]. It's probably no more profound than that."

Eighteen months later, though, sitting at a table in his home studio in Notting Hill, Brian Eno, a man not given to exaggeration, will describe a song that "was hatched almost fully formed in a breathtaking few hours" in Fez as "the most amazing studio experience I've ever had". Which is saying something. That song is called Moment of Surrender, a thing of complex rhythmic beauty and cumulative power, that, as Bono will later point out, occupies the same place on No Line on the Horizon as One did on Achtung Baby. That is to say, it is the emotional centrepiece of a big, overloaded, creatively risky record. "Apart from some editing and the addition of the short cello piece that introduces it," says Eno, "the song appears on the album exactly as it was the first and only time we played it."

Later, too, Larry Mullen, who in the past has been less than enthused by U2's more experimental work - he all but disowned the ambient album Original Soundtracks 1, released as Passengers, back in 1995 - will tell me that "the work we did in Fez was the most joyous and liberating part of the whole album process. It was what I had always imagined being in U2 would be about: just playing music for the joy of it with no real end in sight. It was chaotic at times but even the chaos was creative. You can lose sight of that sometimes with all the other stuff that now comes with being in U2."

(Later Bono will say of Morocco: "What surprised me was that Larry went with it. I was waiting for the eyes to roll. But they didn't. I mean, most of the time, it's hard enough to get Larry to come over to the south side of Dublin.")

On the second day I spend in Fez, I catch a dramatic glimpse of "all the other stuff that now comes with being in U2". In the afternoon, Paul Allen, co-founder of Microsoft and the 41st richest man in the world, drops in on rehearsals. And he brings his band with him; four middle-aged guys in sailing gear and baseball hats. A couple of them even strap on guitars, and, for a brief moment, it looks like they might sit in on a U2 rehearsal. Then, after an impromptu burst of bar-room rock, they depart, grinning like teenagers.

That evening, Allen and his buddies reappear at a dinner that Bono is hosting on the hotel balcony for Queen Rania of Jordan, who, the following afternoon, will also drop in on a U2 rehearsal. "The elegant Jordanian Royal", as she is referred to in the tabloids, sits on a couch, looking, well, elegant and regal, while Bono sings one of the quieter songs the group have been working on. It's a long way from bottom of the bill at McGonagles and the last bus home to Ballymun Avenue, that's all I can say.

Both Queen Rania and Paul Allen are major players in the world of high-end global philanthropy, which is one of several rarefied socio-political networks that Bono now inhabits as part of his other gig: the world's most well-known campaigner for African debt relief. There were moments in Fez, though, when it was difficult to tell which one was now his day job.

After dinner, I chat with Mullen over a few cold beers. "There is a danger," he says, when I mention how strange it was to witness Bono's two worlds colliding in such a spectacular fashion, "that people start to perceive U2 as a part of the Bono show. Now, I admire and support everything he does," he continues, "but that is categorically not the case."

When U2's sojourn in Fez ends a few weeks later, Bono jets off to a Ted [Technology Entertainment Design] conference in Tanzania, while the rest of the band head back home to Dublin.

U2 have now been together for 33 years now, an eternity in pop terms. For the past 22 years, since their fifth album, The Joshua Tree, pitched them into the ether of global rock stardom, they have been the biggest rock group in the world.

In their time at the top, the band have seen several generations of contenders to their throne come and go, including the Clash, the Smiths, Nirvana, the Stone Roses and Blur. For a moment, it looked like REM, then Radiohead might steal their thunder, or even Oasis. As if... Maybe the Kings of Leon or the Killers may yet step up to the challenge, but let's just wait and see. Thus far, love them or hate them, U2 have been unassailable. No other rock band has lasted longer, nor made such consistently good, and often challenging, rock music, nor staged such epic and technologically cutting edge shows.

What is most intriguing - and, to their detractors, infuriating - about U2 is that they succeeded by ignoring, indeed breaking, most of the unwritten rules of rock stardom. They didn't - with the exception of the pre-rehab Adam Clayton - do sex or drugs and, as their critics pointed out, neither did they really do rock'n'roll. They were not rebellious, nor angst-ridden, nor did they trade on adolescent alienation or anger. Instead, they did joy. And spiritual joy, to boot. This made them unfashionable in Britain, the irony capital of the world, where sincerity, especially sincerity tinged with spirituality, is seen, at best, as uncool, at worst as downright embarrassing.

"One of the reason's for U2's longevity," says Brian Eno, "is that they are not in music for entirely selfish reasons. I don't want to make them appear as evangelists, which, of course, they were seen as by some sections of the music media in the early 80s, but I do believe that they really think that what they do serves some greater purpose than simply filling their bank accounts."

Initially, I had little time for U2, their songs, their haircuts, their Christianity. My epiphany occurred when I was sent to Rome by the NME in the summer of 1987 to interview Bono after the first gig of their European tour - The Joshua Tree tour. Put simply, it was a revelation: a rock group whose music made sense in a stadium, whose songs retained - and inspired - a kind of communal intimacy in a crowd of 60,000 people. And, boy, did Bono work that crowd. He was one part rock star, one part showbiz trouper, one part preacher man. In America, where cool is not such a reductive currency, U2 were embraced with open arms. The rest, as they say, is history.

By Achtung Baby, as Bono famously put it, they "discovered that irony was not the enemy of soul". The Zoo TV extravaganza was, and remains, the most technically innovative - touring rock show of recent times. And anyone who still thinks U2 don't have a sense of humour obviously missed the Pop Mart tour, where they emerged nightly out of a giant lemon dressed like some postmodern version of the Village People.

This is the version of U2 that I prefer, the one that challenges our preconceptions of U2. It has not been around for a while, but now it has popped out of the closet again on (most of) No Line on the Horizon, which is a world away from the two traditional sounding, good-but-not great albums that preceded it. They seem to me, at times, to be the last of something: the last rock band that insists rock music has some greater meaning at a time when the form seems dogged by a lack of cultural resonance.

Hanover Quay Studios, Dublin, June, 2008

For the past year, the group have been working in fits and starts in New York, the south of France and Dublin. Steve Lillywhite is now on board as a co-producer alongside Eno and Lanois. When I arrive, he and Lanois each are working on separate versions of a song called Sexy Boots, the title of which, after much discussion, will be changed to Get Your Boots On. It will subsequently become the first single: a Zeppelin riff welded to a bubblegum pop melody; surprising, sexy, sinuous. Later, Bono will play me three other almost finished songs: Unknown Caller, No Line on the Horizon and Chromium Chords, which will later be re-titled Fez - Being Born. The songs, on first hearing, sound dense and elusive. You can hear Lanois and Eno's presence on all of them. I try to take them in as Bono talks - and sometimes sings - me though them.

The album has developed, he says, into a kind of "fractured journey, a physical journey from Paris to Tripoli via Cadiz, but also an emotional and psychological journey". It sounds, I say, like a concept album. "Don't even mention those words," he says.

That evening, as we sit down for dinner, more songs are played on the sound system: Magnificent, the most U2-sounding song, epic and soaring; Cedars of Lebanon, a more intimate song delivered in a half-spoken style; Breathe, which sounds like a page torn out of the Dylan-on-amphetamine songbook ("Nine o nine, St John Divine on the line, my pulse is fine, but I'm running down the road like loose electricity"). He seems fired up on the possibilities of where this album is going.

"I just got tired of the first-person so I invented all these characters; a traffic cop, a junkie, a soldier serving in Afghanistan."

As Moment of Surrender starts, he jumps up and sings along to the hallucinatory lyrics. "I was speeding on the subway/ Through the stations of the cross/ Every eye looking every other way/ Counting down till the pain would stop." A spiritual epiphany? A junkie's final fall from grace?

Before I can ask, Bono has returned to the table, his laptop open, and is reciting what sounds like a Beat poem. It namechecks Keats and Shelley, St Augustus, a neon Jesus and "the gods of Apollo and Zeus", and there's a line about "tourists with bad breath" and "campaigners against bad debt". There's reams of this stuff, surreal, freeform verse that makes a certain kind of Ginsbergian sense. It does not make it on to the album, but may surface in future live shows if the spirit moves him.

Around midnight, taxis are called, and I head for the Shelbourne Hotel for a late drink with Daniel Lanois. He looks tired. "It's taking longer than we thought," he says, sipping on a beer and a brandy chaser. "They always go the extra mile. They're intense people. I'm intense myself."

Lanois is an old-school rock'n'roller who has worked with Dylan and the Neville Brothers, and who likes to keep things loose and atmospheric. He appears laid-back, but is anything but. I tell him something that Bono had said about him earlier - "Danny's attitude is, 'It's going to be a great album or somebody is going to die.'" He laughs and raises his glass. "That about covers it, Sean. I ain't here for the money, man. None of us are. It ain't about a salary, it's about making a fucking great U2 record."

Has that been difficult this time around?

"Kind of, but, then again, U2 albums have always been difficult."

A few months later, in September 2008, it is announced that the release date of the new U2 album has been put back from November to March. One nagging question hovers unanswered over the postponement: is Bono's other life as a campaigner and activist leaving him too little time to give himself fully to U2?

"When Bono's there, he's there," The Edge tells me later. "He still gives huge amounts of his time and energy, but his life is undoubtedly different now." Larry Mullen concurs. "I can tell you categorically that all the other stuff is not affecting his work. He has boxes of lyrics, great lyrics."

Have his absences impinged on the making of this album?

"Well , when you are four guys working together and one of them is away a lot, you miss that chemistry, and you miss his input. But there's no sour grapes there. We get on with it. We work, you know, U2 works."

Later, I ask Bono the same question. How does he find the time for U2 these days? He takes a deep breath.

"When I'm with U2 doing U2 work, they have me 100% or we would not be here now and we certainly would not have made an album like this one. Look, my day is long. My creative life is over at midday. But, you know, I get up very early. Plus, I don't go out and set fire to myself on a regular basis. I still do it on the odd Friday night, but not the way I used to. I give my time to my family, my band, and my interest in the wider world. It all seems to be fuel for me. My engine seems to be working better these days."

At a time when celebrity is a degraded currency, Bono has turned his fame into the ultimate calling card for his activism. It has helped opened doors from the Vatican to the White House, helped ensure unprecedented amounts of aid and debt relief for Africa, helped save and transform countless lives that would have been lost for want of retroviral drugs, and it has led to unlikely alliances, maybe even enduring friendships, with Nelson Mandela and Bill Clinton, but also with George Bush, Tony Blair and Nicolas Sarkozy.

In all of this, Bono has not only rewritten the rules of rock stardom, but willed himself into a place where no other rock star has gone before. It has been a high-profile tightrope walk that has earned him much praise and much negative criticism, even scorn. "There are probably more annoying things than being hectored to about African development by a wealthy Irish rock star in a cowboy hat," thundered the travel writer Paul Theroux in a very public broadside a few years back, "but I can't think of one at the moment."

I ask Bono if he can understand why a lot of people, myself included, not to mention his drummer, found his perceived closeness to Blair and Bush hard to take? He sighs.

"I can understand that, for sure, but the results speak for themselves. I can take it on the nose from everybody, including my own band, but by the time he leaves the White House, George Bush will have trebled aid to Africa. We are into him for $50bn."

SOH So, is it part of the deal that you then don't criticise him about anything else he has done - the war in Iraq, say, or Guantanamo? Morally, that's quite a tricky trade-off.

Bono "No, it's more that I don't make a song and dance about my criticism. Everyone in the White House knows where I stood on the war. In the run up and when it was just about to happen, I had many conversations where I expressed my feelings. But I felt I had to focus on this one thing which was, don't make a deal on extreme poverty. Make it truly colourless politically. It was the power of one clear idea. And it succeeded. And it was very, very difficult, and there was a lot of hand-holding, hours and hours, weeks and weeks, meeting after meeting after meeting, trying to get people not to play politics with the world's poor. And for me to alienate people who, to be fair to them, were often sending their sons to Iraq I just felt, I don't want to be shouting my mouth off about this war when really I have a chance, along with other people, of achieving for the first time broad political consensus on this one hugely important single issue of Africa and aid."

He reaches for his drink and shakes his head.

"But you're right, you're right, you're right...I mean, you know me, and you know how difficult it is for me to shut up about anything."

SOH What intrigues me, though, is this tricky place you are in, which is quite unprecedented in pop star terms. You have used your celebrity to go into the world of political activism, but also the world of corporate wealth and the super-rich, and all in the cause of fighting poverty. It's a tricky place for a rock'n'roller to be.

Bono "I know, I know. It's dangerous. And it worries Larry, and it worries the whole band, if truth be told. But, you know, here's the thing - they thought, all of them, Larry, Edge, Adam, that my campaigning would sink the ship."

SOH The U2 ship?

Bono "Yeah! They thought that the rotten tomatoes world rain down and people would not be able to hack it. That was before they started throwing them themselves [laughs]. They thought, one, that it would distract me. And more than that... They were just not into it at all.

SOH All three of them were against it?

Bono "Oh, I think so. I mean, Edge pleaded with me right at the start not to meet Bush. Five or six years ago. They all did."

SOH Initially, Bush wasn't so keen on meeting you, either. So how long did it take you to get a meeting with him?

Bono "It took a while. He just did not want to meet me at first."

SOH How much difference does your faith make in these situations - your Christian faith?

Bono "A big difference. I mean, I had been a serious scourge of the religious right, and particularly the evangelicals, for their inaction on Africa. And, to be fair to them, after taking a serious beating they did get up and do something, and that gave Bush cover on his right flank."

SOH Did you ever get into theological debate with Bush?

Bono "Oh yeah! I wouldn't talk about it, but yeah."

SOH OK, I have to ask you this one. Did you ever pray with him?

Bono "Whoaah! (He reaches for his drink.) But I didn't inhale [laughs]. Look, I would always use the scriptures to argue my corner. There are 2,103 verses of scripture pertaining to dealing with the poor. That helps, and it also helps to know that.

"Boy, did my days of Bible study come in handy! And, by the way, it's an offence to me that religious people can close their eyes to this stuff. It's just really not allowed."

SOH From where I'm sitting, though, a lot of the people that you are bargaining with, and who are undoubtedly helping save lives in Africa, have also, by their actions elsewhere, shown a blatant disregard for human life on a grand scale. Surely, that, too, is their legacy?

Bono "Look, it's appalling and shocking and not ever excusable, the waste of human life. But on our issues, all I can say to you is that there are 29 million children in Africa who were not going to school and who now are. That's just in seven years. Now, that's not an excuse for a wrong-headed adventure. It's not an excuse. But I don't believe Tony Blair is evil. I know him enough to know that he is a sincere and serious person who would in any unserious way make those decisions and, though I disagreed with those decisions at the time, I think it's really simplistic to think that he is anyone's poodle."

SOH So, hand on heart, when you are dealing with these political heavyweights, do you ever think you are being played?

Bono "I don't care if I get the results. You have to judge me only by the results. If there were no results and you saw a picture of me hanging out with George Bush or Tony Blair or whoever, that would be a different matter. But if you see a picture of me and Bush and two years later you hear people saying 'How on earth did a conservative administration start the largest response to the Aids emergency yet?' I understand why people threw tomatoes at me at the time but even the worst critics have stopped."

SOH There must be moments in all this when you stop and think, this is too surreal. What the hell am I doing here?

Bono "All the time. I mean, there's me and Bob [Geldof] at the G8, and there's the Japanese and their plane is parked over here, and Air Force One is parked over there, and there's the French, the Italians, the Russians, the leaders and a tight coterie of advisers. And then there's fucking Bill and Ben the Flower Pot Men who've somehow been let in. And Bush is going, 'Hey, Bono!' And there's Sarkozy and Merkel, who has given us the keys too because she's heard from Tony Blair that we were the right people to let in. Will we see the likes of it again? I don't know. It still feels mad to me, how that even happened."

London, early December, 2008

The album now has a title, No Line on the Horizon, which is beginning to sound like a self-fulfilling prophecy. "This is definitely the last week of recording," says Adam Clayton, who is wandering the studio corridor when I arrive, espresso in hand. "But then again, last week was definitely the last week of recording, and the week before that."

Downstairs, in a room adjacent to the big wood-panelled studio in which the Rolling Stones recorded Sympathy For the Devil, Bono, The Edge and Lillywhite are working on a single verse of a song called Stand Up Comedy. They have been working on the verse for hours, the song for 16 months - since Fez, in fact, when it was called For Your Love - but something is still not right. Lillywhite, a man possessed of seemingly endless reserves of effervescence, is hunched over a vast mixing desk, doing whatever it is producers do with sound levels, textures, equalisers.

"OK, let's try it one more time," he says, spinning around on his chair and signalling to Bono, who stands up and reaches for a cordless microphone. Lillywhite flicks a switch on the console and a thunderous guitar riff explodes out of the studio speakers. "Stand up, step out on the wire..." howls Bono, his face contorted with passion or, perhaps, pure frustration.

Lillywhite cuts the music and, within seconds, replays the take. Bono shakes his head as the playback blasts out of the speakers. "Annoying Bono just reared his head on that one," he says, reaching for a bottle of water. The other two crack up laughing. "He was definitely hovering," says The Edge.

Something gives in the room, an indefinable, but palpable, diminishing of tension. Still chuckling, Lillywhite re-cues the backing track, and Bono stands up to give it all he's got one more time again. It sounds like he has nailed it. "That was magic, Bono. We're done!" says Lillywhite, looking relieved beyond measure. He replays the take. Bono looks at The Edge. There is a long moment's silence. "I'm not sure," says The Edge, quietly. "I'm not sure. It sounds fine but it doesn't feel right."

When I pop upstairs to grab a coffee a few hours later, they are still working on the same verse.

"It can be frustrating at times when they sometimes take a song and work it into the ground, then work it back to life again," Brian Eno will tell me later. "That's what happened with Stand Up Comedy. I was thinking the other day that Edge has probably heard that song more times than even the most dedicated U2 fan ever will."

Songs - and characters - still survive from Dublin, but the album's narrative arc has since been jettisoned, along with two other songs, Winter and Every Breaking Wave, in the last few frantic days of recording. We will have to wait a while longer for the great U2 concept album. "It was painful to me in a way to let go off the original idea," Bono says, "but by then, I was getting harassed on all sides by everybody. The thing is," he says, grinning, "we came up out of punk rock. We were never going to make a concept album. We were never going to do the prog rock thing and let the plot get in the way of the songs."

It will be interesting to see how No Line on the Horizon, which announces its otherness in its minimal sleeve - one of Hiroshi Sugimoto's zen photographs of a grey sky merging into a grey sea - will be received by the group's traditional fanbase, that huge global constitution for whom U2 will always mean big, stirring songs borne on thudding rhythms and chiming guitars. Instead, they will find a big, dense multilayered album that is the sound of a band reinventing itself once more.

Six weeks later, I enjoy two final conversations with Bono, the first in the corner of the sedate bar of the Merrion Hotel in central Dublin. Remarkably, after the dogged months of studio toil, he looks fit and healthy, trimmer than of late and, when the shades eventually come off, bright-eyed. The buzz of excitement that had spread though the establishment on his arrival has subsided but a waiter hovers in attendance, bearing drinks, then snacks, then, just to be on the safe side, soft drinks.

SOH You said back in Fez, "This record is about the world, and the desire to get out of it, to escape it." Can you elaborate on that?

Bono "Did I say that [laughs]? Were we talking about the notion of peripheral vision, which, I think, is central to the understanding of this album."

SOH I'm not sure but go on.

Bono "Well, first up, it's a very personal album. These are very personal stories even though they are written in character and, in a way, they couldn't be further from my own politics. But, in the sense of the peripheral vision, there's a world out there. As the old blues song goes, a world gone wrong. You can feel it just at the edges - the war in Iraq, the dark clouds on the horizon. But there is also a deliberate shutting out of that in order to focus on more personal epiphanies."

SOH Why did you choose to do that?

Bono "I think because I'm so very much out in the world most of the time, whether the world of commerce, of politics, of activism, whatever. So I have learned to really value the interior life of being an artist and a writer and being in U2. It's become a very private and special place, the time when I'm working with the band. The songs have become more intimate. I wanted to get to an intimate and inner place. I want to get away from subject and subject matter into pure exchange. Not even conversation. Often, it's just like grunts or outbursts. When I think of Moment of Surrender, it's just there! Or Breathe [starts singing] '16th of June, nine o five, doorbell rings...' You're right there in the middle of this outburst. For somebody who spends a lot of time in the exterior word, this album is very much about the interior world."

SOH OK, I'm going to throw some lyrics at you.

Bono "Fire away."

SOH These are from I'll Go Crazy If I Don't Go Crazy Tonight: "Every generation has a chance to change the world/ Pity the nation that won't listen to you, boys and girls."

Bono "Well, that is building up to the next line, 'The sweetest melody is the one we haven't heard.' That's just a nice thought. The solution to the problems we find ourselves in will have to be found by the new generation but often the new ideas just aren't listened to. That lyric is meant to be playful, by the way, not earnest in any way. There's a lot of mischief on this record.

SOH Was that one written with an eye on Obama coming into power?

Bono "Of course! The amount of U2 fans who supported him! The young U2 fanbase were really active in the campaign. Though the One campaigners are from every political colour, an enormous amount of them were also campaigning for Obama."

As we speak, the Obama inauguration is just a week away, and, in a few days, U2 will fly out to Washington to help kickstart the celebrations.

SOH Do you think that Obama's team is equal to the challenge: intellectually big enough to take these huge problems and tackle them on a conceptual level?

Bono "I do. And in a way that has not even been written about."

SOH You know this or you're projecting?

Bono "I know. We're already beginning the conversations."

SOH So you're hopeful? Even as the world freefalls into global financial meltdown...

Bono "Yes. Totally. It's a scary and an amazing time. Look, the world is waking up again. Not to get too grandiose on your ass, but there are shifts that always happen after a major crisis. So, after the First World War, the League of Nations; after the Second World War, the United Nations. The IMF, the World Bank, all came about after periods of crisis. And after 9/11, the Iraq dabacle, and the market meltdown of the last year, I think this is the moment when actually everything is up for grabs. It's like Bob Dylan says on Brownsville Girl [he breaks into a Dylan impersonation]: 'If there's an original idea out there right now, I could use it [laughs].' And there are original ideas out there, that's the thing."

SOH OK, on I'll Go Crazy...., you also sing, "The right to be ridiculous is something I hold dear."

Bono [Laughs] "That's me, That's not an in-character song. I mean it in the literal sense [laughs]. It's actually very important. One of the things I think we've been good at is not letting people put us in any kind of pious light. That happened to us for a while in the 80s and we never want to go back there. I'm always shocked that people are so shocked when they discover the silliness that is an everyday occurrence with U2. It's the final blow to people who can't stand us. That we seem to be having a better time than everyone else as well. It's like, it's not enough not to have broken up, to have made some hopefully inspiring music over the years, but also to be having a lot of fun. The mischief is part of our story and it isn't represented or read about. That's one of the reasons that people do a double take when they see me staggering out of a pub in Dublin at 4am. It can't be Bono, can it? Nah."

SOH So it irks you that people don't seem to get that side of you?

Bono "It takes the sexiness away from you, for a start. And the aliveness."

The conversation spirals off into illuminating territory, touching once again on the Christian faith that is the key determinant of both his music and his activism. It is a subject he does not often talk about, he says, because it inevitably gets reduced or trivialised, and "it leaves you open to being accused of being a hypocrite, especially if you are of the hopeless variety, which I am. I haven't broken all the commandments," he adds, laughing, "but I've wanted to."

He says that a lot of people he most admires are non-believers. Bill Gates. Warren Buffett. "People who are prepared to spend their entire life's fortune trying to make the lives of people they don't know a lot better. These people are more Christian than the Christians. Zealotry and certainty are worrying for me. Love keeps religion from zealotry."

SOH So without love, it becomes another kind of fixed ideology?

Bono "Yeah, that's right! Anyway, there's loads of pops in there about zealotry, religious and otherwise, and you're the only person who's picked up on this in the lyrics. I mean, 'Stop helping God across the road like a little old lady.' Come on?"

SOH That's a pop at the militant atheists.

Bono "And at myself. I mean, I have a bit of it, myself. I have a bit of the helping God across the road like a little old lady."

SOH Not as much as you used to...

Bono [Laughs] "No, but I do have a bit of it. People like myself, all activists, can be guilty of thinking that, because these are matters of life and death, we have an excuse to be fanatics. You have to know when you stop. At least, I do. That's why a lot of anti-poverty campaigners are so annoying. [laughs]. Me included. That's why people see me and they go, 'That fucking Bono, get him outta my face.'"

The following week, Bono and I have one final conversation, and I ask about the album's last lines: "Choose you enemies carefully, 'cause they will define you/ Make then interesting, because in some ways they will mind you/ They're not there in the beginning, but when your story ends/ Gonna last longer with you than your friends."

Bono "Yeah. Yeah. They're are going to be closer than your friends. They are going to shape you."

SOH Are you singing from experience here?

Bono "In a way, I guess. I think one of the things that has set our band apart is the fact that we chose interesting enemies. We didn't choose the obvious enemies - The Man, the establishment. We didn't buy into that. Our credo was: no them, there's only us. Think about it. Every other band was us and them. The Clash, our great heroes. Then U2 arrived and it was no them, only us.

"What that means is that we picked enemies that were more internal - our own hypocrisy. The main obstacle in the way of our band we always saw as ourselves and our limitations. We never blamed the record company. We never blamed the radio [laughs]. You never heard that from us in 25 years. It was always, can we be better? Can we make the song better, the show? What you're really dealing with then are the obstacles to realising your own potential. They are nearly always of a psychological, if not a spiritual, nature. The spectres that hold you back, they were our enemies. It was always, 'You're supposed to be in a rock'n'roll band. You're supposed to be rebellious, but you don't rebel against the obvious.' And we'd go, 'No, we don't. That's the point.'"

SOH In that way, your success ran counter to the course of rock'n'roll. You sang of the joy as opposed to the angst.

Bono "Yeah. I mean, the mark of succeeding for us is.... erm, let me try and get this right because it's important. Joy, for me, is the spilling over of a life well-lived."

"But to get back to the last lines of the record. We were talking about peripheral vision at the start of this conversation. That's the theme of the record. And, in one sense, it's a very tough-minded theme, even, some have said, bleak, but I don't think so."

SOH So, hang on, can you pinpoint that theme, break it down for me?

Bono "It's like, well, you think of the heartache of the invasion of Iraq, or the heartache of taking on heroin, which we've known from friends who have taken that drug, and you think of the wasted energy in a life that comes from just taking on the wrong fight. It could be with your lover. There it was and you blew it. You just didn't know what you had. That's why I love the opening of the album - No Line on the Horizon. There's no end in sight. It's infinity, it's optimistic. [sings] 'I know a girl who's like the sea.' The sea and the sky become the same colour and you lose the horizon line as it disappears into infinity. Infinity is a great place to start.

"You know, it's like that thing that people said about U2, that most bands start off writing about girls and end up writing about God, but we started off writing about God and ended up writing about girls. But we found the God in the girls, that would be my retort."

SOH OK. I need to process all this stuff.

Bono "Anyway, that image is very optimistic - no line on the horizon, whether about a band, a girl, the future. Life itself. It's like I say, the future needs a big kiss."

The future is another question for another interview. How long can U2 stay meaningful? Where will rock's greatest adventure end? For now, there is enough material left over from the sessions for an album that, Bono says, will be released before the end of the year. It will be "a more meditative album on the theme of pilgrimage". Despite the slog, he retains his sense of humour.

SOH The funniest line on the album may be on Stand Up Comedy: "Beware of small men with big ideas." Are you referring to yourself?

Bono "Yeah! For sure. That is the funniest line. Of course. It's saying, stand up to rock stars. That's about choosing your enemies, too. What are you gonna stand up for and what are you gonna stand up against? I love the notion of standing up to rock stars. Because they are a bunch of fucking megalomaniacs [laughs]. If you don't laugh at the end of that line, there's no hope. When I wrote it, I burst out laughing."

Bono calling: Who's on his speed dial?

Bill Gates
Palled up with Bono during work on poverty in Africa. In 2005, the Microsoft billionaire and his wife were named Time magazine's joint "Persons of the Year", alongside - who else?- the U2 singer.

Jeffrey Sachs
Another ally in the mission to help Africa, economics guru Sachs wrote The End of Poverty, which argued it could be eradicated. Bono pitched in with the foreword.

Oprah Winfrey
The Queen Of All Media is a player in Bono's Red charity, which combats HIV and Aids. In 2006, the pair publicised its work by going shopping together in Chicago.

Tony Blair
Speaking at the 2004 Labour party conference, Bono invoked the Beatles, calling Blair and Gordon Brown the "John and Paul" of global development.

George Bush
A controversial friendship, although endless pestering of the least popular president in recent memory did produce a $9bn rise in emergency relief for Aids in Africa and the Caribbean.

Barack Obama
The 44th president used U2's City of Blinding Lights during his campaign, and, although he favours no one celebrity at the moment, inviting the band to play at his inauguration suggests Bono remains very welcome at the White House.

U2: A life in reinvention

1981: Finding God
The first inkling they were more than just another post-punk band - the religious themes of October were the result of the band falling in with Christian fellowship Shalom.
Key track: Gloria

1983: A little bit of politics
The next stage of their desire to use rock as a force for good, with War's Sunday Bloody Sunday wading in on the Northern Ireland Troubles.
Key track: Sunday Bloody Sunday

1987: Discovering America
During the Reagan era U2 investigate the US's complex national identity on The Joshua Tree (working title: The Two Americas).
Key track: Bullet the Blue Sky

1991: Irony, anyone?
Even Bono decides he's bored of himself. Cue wrapround sunglasses, alter egos The Fly and MacPhisto, plus their best album, Achtung Baby.
Key track: The Fly

2004: Back to basics
Post White Stripes, rock'n'roll is back on the agenda. U2 want in on How to Dismantle an Atomic Bomb, if only to offset Bono's second job as an international statesman.
Key track: Vertigo

• No Line on the Horizon is released on Mercury on 2 March. To read the first review of the album, click here.