Neil Tennant, Pet Shop Boy and perceptive analyst of pop culture, was excited. But he was also concerned. 'When I see someone like Brandon Flowers who has the appetite, and possibly the talent and looks, to be a star, I find that enthralling,' Tennant said recently of the Killers' singer. 'I'm worried, though - and I hope he's reading this - that he's grown a beard. It means he's saying, "I'm not pop. I mean more than that."'

When I tell him this, Flowers is mightily amused. 'Hah hah hah! Man,' he whistles as he chews over a fatty steak in a Japanese joint in Los Angeles. Growing up in Las Vegas and small-town Utah, a fat kid and a Mormon to boot, Flowers dreamt of escape. And of change. He was an Anglophile with a burning love for New Order, the Smiths and Pet Shop Boys. The lyric that meant most to him above all else was from the latter's 'Being Boring': 'I never dreamt that I would get to be/The creature that I always meant to be'.

These days, Flowers is singer with the one of the biggest new bands in the world: the Killers sold 5m copies of their 2004 disco-pop debut Hot Fuss, and may well exceed that tally with the massive-sounding follow-up Sam's Town. He's a slight 25-year-old with a boyish frame. The Rolling Stones' Some Girls T-shirt that he's wearing this lunchtime on Sunset Boulevard, and will still be wearing tomorrow afternoon in Las Vegas, hangs off him shapelessly.

He's jittery with nervous energy and looks more like a geeky student than a rock hero. Over two days in his company, I see him pained by the smallest details. Offstage, his bandmates - wise-cracking, mutton-chopped drummer Ronnie Vannucci; big-haired guitar wizard Dave Keuning; lanky and enigmatic (ie teetotal and quiet) bass player Mark Stoermer - have more rock'n'roll charisma. But give a microphone and a stage to Flowers, five years his co-Killers' junior, and the lights snap on. You can't take your eyes off him.

Yet success hasn't brought him contentment and surety. He has a yuk-yuk laugh that makes him sound like Jimmy Stewart in It's a Wonderful Life. When asked about his Mormon faith, he'll stumble and falter. He's aware of how uncool it is to talk about your religion. But he talks nonetheless. He can't help himself. He's polite and funny, but honesty and directness shoot out of him. This can make him not the easiest fellow to feel comfortable around. With Brandon Flowers, everything comes out for all to see.

Notably, as Tennant observed, his hunger for the spotlight. Laurels are not there to be rested upon. Inspired by the mouthy frontmen that so appealed to his teenage self, Flowers maintains a withering disdain for those (smaller) groups he sees as coming up behind the Killers - and for those bigger outfits within range of his hyper-aspirational band. He thinks fast-rising American band Fall Out Boy are dumb; he thinks Radiohead's Thom Yorke is squandering his talent: 'He should feel grateful that's he's [been] given the gift to write pop songs - which he needs to write again!' And even now that he's on speaking terms with Elton John, Morrissey and Bono - and now that he has sales to match his ego - this disciple of music cares what his heroes think.

'He's a smart man, Neil Tennant is,' Flowers says contemplatively. 'I do feel like we're definitely still pop. And we've never been ashamed of that. This is just a phase. I've never been able to grow a moustache before! I'm upset that Neil doesn't like it!'

Onstage and in photographs, the Killers are indeed rocking a bold new look. Where once they were called 'America's Best British Band', for Sam's Town they've returned to their roots. The album is named after a casino in Las Vegas built in 1979 (prehistoric in Vegas terms) by a 'good ol' boy' named Sam Boyd. The sleeve, black and white and desert-y, was shot by Anton Corbijn, the photographer responsible for the iconic imagery of U2's The Joshua Tree.

'Initially they wanted a chic, gypsy look,' says Corbijn, who also made the video for Hot Fuss's 'All These Things That I've Done' and asked the Killers to cover Joy Division's 'Shadowplay' for his upcoming Ian Curtis biopic Control. 'Out of those discussions [for the sleeve] came these elements of faded glory.'

The tuxedos, pastel jackets, eye-make up and skinny Dior-meets-New Romantic stylings that marked the Killers out as a different kind of American guitar band? Consigned to the great rock'n'roll dressing-up box in the sky. They've been replaced by facial hair, bootlace ties and waistcoats. There's tumbleweed, gathered by a roadie, on the stage. This 'phase', as Flowers calls it, has taken the Killers from the 1980s to the 1880s, from Hollywood to Deadwood.

'I think we're embracing a bit more of our own culture,' affirms Flowers. 'It's fun - it's all a part of getting ready for a show. Just like it was when it was a more glamorous-looking version of the Killers.'

But their mentors, Flowers declares immodestly, shouldn't be alarmed. 'We haven't lost the pop sensibility,' he smirks. 'He'll be proud of [new song] "Read My Mind", Neil will.'

Brandon Flowers, the youngest of six, was born in Las Vegas. When he was eight he moved with his family to Utah, first to Payson (where the film Footloose was shot) then to Nephi (a tiny community named after a Mormon prophet). His parents had lived in Vegas for 40 years; they wanted to get out of the 'rat race'. Flowers 'never really got over' the culture shock of the move, although 'I liked the freedom of being a kid in a small town. My mom didn't have to worry about kids getting kidnapped.'

But he was fascinated with old-school Las Vegas - 'with Frank Sinatra and the glamour and having a Cadillac' - and when he was 16 his parents let him move back, to finish high school and live with his cousin, who was the same age. After graduating he worked in casinos and shops, and began looking for like-minded musicians.

After spotting Dave Keuning's advert in two Las Vegas weekly papers, Flowers and the guitarist bonded over an enthusiasm for Oasis, and a desire to be huge. Keuning had come to Las Vegas from small-town Ohio to start a band. He'd had 18 months working in other bands. His first impression of the local youngster:

'I thought he had weird shoes,' says Keuning. 'He had the same shoes Oasis had - Clarks!'

Ronnie Vannucci and Mark Stoermer, both Las Vegas natives, had also done time in local bands. They met Flowers and Keuning via bar-circuit contacts, and in August 2002 the four played their first gig together. 'You could tell that there were good songs there,' recalls Stoermer of meeting Flowers and Keuning. 'And that special something it takes ...'

Vannucci remembers that even when their songs were 'little dwarf versions of what we have now, Brandon wasn't afraid to just get up there and just do it. You need that when you're trying to get something off the ground. As far as the drive goes, Brandon was never half-assed.'

Fiercely competitive, Flowers has been trying to catch up with his five significantly older siblings since he was born - he was a late baby, 'a surprise!' - notably his brother Shane. Shane is 13 years older than Brandon, born on the 4th of July, and a much better golfer than the youngest Flowers could ever hope to be.

As an adult, the youngest Flowers had much to prove. In the case of Sam's Town, this means proving that these four disparate characters from Las Vegas can take the momentum generated by their Brit-disco first album and turn it into a follow-up that evokes Bruce Springsteen, U2, even ELO and Queen. For the Killers, born of America's wide-open spaces, size is now everything.

Even on the small stages the Killers are playing this weekend, Flowers performs like he's in a stadium. Here he is, prowling the stage of the tiny, legendary Troubadour club in LA, leading the crowd in a rapturous singalong of 'I got soul/But I'm not a soldier', the chorus from 'All These Things That I've Done' - the declamatory line, sung by a band all nattily dressed in white, that reverberated loudest at last summer's Live 8, and which was stolen by Robbie Williams on the Hyde Park stage.

Here he is again the following night, halo'd by fairy lights and surrounded by tumbleweed in the Empire Ballroom in Las Vegas, grinning like a kid as the hometown crowd whoops and punches the air and the Killers power through 'Read My Mind', an ode to Vegas.

With characteristic hyperbole, Flowers has already claimed that Sam's Town is 'one of the best albums of the past 20 years'. At his home in Henderson, a well-to-do community outside Las Vegas, he recently shut himself in his car in his garage and treated himself to a playback of the whole album. He can now confirm that it is indeed a corker. 'I can back up my big mouth,' he beams.

On Sam's Town, the Killers concern themselves with dramatic stuff: home and heartache, life and death, right an d wrong. 'Bling (Confession of a King)' is the victorious story of Flowers' dad forswearing - overnight - alcoholism and Catholicism to become a Mormon when Brandon was five. 'Higher and higher/We're gonna take it/Down to the wire,' sings Flowers in another arms-aloft Killers chorus. 'We're gonna make it/Out of the fire.'

More dirty family laundry is aired in 'Uncle Jonny'. It's about how his mum's brother survived a cocaine habit, a paranoid obsession with George Orwell's 1984, and the conviction that aliens were coming to steal his semen. So Uncle Jonny decided to shoot his testicles off in the bath. Happily, he missed, and shot himself in the groin instead. He's all right now. 'There's a real inspirational feeling by the end of the song, I think. You're pulling for Jonny. That's a good thing, having faith in people.' It's a mark of his single-mindedness - some might call it selfishness - that Brandon Flowers didn't bother checking with Jonny or his dad whether he could put these songs on Sam's Town.

The anthemic 'Why Do I Keep Counting?' - this album's 'All These Things That I've Done' - is more personal still. It's about Flowers' flying phobia, and his fears of mortality. 'Will I live to have children?' he wonders. He's learning to manage the fear via weekly stints with a therapist in Las Vegas. On planes he sits listening to tapes of their sessions, clenching and unclenching. Unfortunately, that phobia is reinforced by another: Brandon Flowers has a problem with the number 621, the date of his birthday (21 June). If he happens to be driving at 6.21pm he's extra careful. Once he had to fly to Glastonbury on his birthday. 'That was a real mess.'

He's in hysterics (the good kind) as he recounts this. But it's deadly serious. It dates back to childhood, when a Ouija board told him he would die on that day. 'It's just stupid, it's not a way to live.'

He sighs. 'You're growing up and you're not afraid of anything. You just exist and have fun and have no worries. I've been given this great position to be in. I feel really lucky. It's almost too good to be true. That started making me feel like it's inevitable that something really bad's gonna happen.'

The title track on Sam's Town mentions attending his grandma Dixie's wake - the first time he's been directly touched by death. But his parents are in their sixties. 'I'm getting older, my parents are getting older. My mom had bad health problems. You just start thinking: my mom's gonna die one day. Ha ha!' He laughs nervously. 'I'd never thought about that.'

He has a thing about death. 'Jenny was a Friend of Mine' and 'Midnight Show', both from Hot Fuss, were about the murder of a girl. He traces this interest back to Morrissey singing about how he loved 'the romance of crime' in the song 'Sister, I'm a Poet'. 'I studied that line a lot. And it's kind of embedded in me.'

On tour in Scotland, the Killers heard the terrible story of Jodi Jones, a 14-year-old from Dalkeith who was murdered by her boyfriend three years ago. They wrote a song, 'Where is She?', from the perspective of Jodi's mum. They played it live a few times, but after an outcry in the Scottish press have decided to shelve the song.

'I felt really bad,' says Flowers. He's sincerely pained by the upset he caused. But as we conduct our second interview, in a dressing room in Las Vegas, empathy seems to be lacking - he seems more aggrieved by the fact that he can't play the song any more. 'Before I was in the Killers, I would have been able to write that song and nobody would have known. But it came from a good place. If I never would have said anything about it, nobody would ever have known and maybe that song would be on Sam's Town right now. It's a great song. It's a shame.'

And there might be other reasons still why he thinks about death. Mormons believe that death is an integral part of salvation - eternal life - and that death is not the end. But salvation is partly dependant on individuals living a good and decent life. When you're in a rock'n'roll band, that can be tricky.

'With any religion there are do's and don'ts,' says Flowers. 'A lot of people think polygamy is involved and it's not. [Or that] you can't drink Coke - that we think we're gonna go to hell if we drink Pepsi. You're not supposed to drink alcohol.'

But you do.

Flowers grins. 'I try not to.'

He's caught between God and rock'n'roll, the sacred and the profane. No wonder holy Bono is such a hero to him. 'Bob Dylan said it best - you can't be Jewish and be cool,' he chuckles, 'and you can't be a Mormon and be cool! But I'm trying my best!'

It's about picking his way carefully through the world of, as he sang on Hot Fuss, 'Glamorous Indie Rock & Roll'. The Killers' most famous line, 'I've got soul/But I'm not a soldier', has always baffled me. Pressed on its meaning, Flowers eventually concedes that it's about how 'you're not always going to be perfect ... For me it was more of a religious thing. Going through this phase in my life of ... believing what I believe and being thrown to the dogs. To the rock'n'roll world ... Because even when I slipped I still believed what I believe.

'I'm trying to find the ideal place. And I'm still looking. I'm starting to get more comfortable with it. I'm a man and I'm attracted to women. You read about, and you have that fascination with, the drugs. There's a certain level that we're kinda expected to, to to ...' The words come out of his mouth with stuttering difficulty.

'To debaucherise [sic], I guess! It's expected of us, almost.'

Has he taken drugs? Flowers opens his mouth to speak, thinks better of it, and waves his hand back and forth over the tape recorder, smiling, uncharacteristically at a loss for words.

Last year Flowers married his girlfriend of five years in Hawaii. She's almost finished her primary-school teacher studies. But now that he has the money, he wants to bring her on tour. He's desperate to have a baby, 'a little Flower child', but Mormonism places an exalted premium on the solidity of family life; one of his cousins gave up a place on the PGA golf circuit because it would mean being away from home too much. Flowers sees how difficult it is for (non-Mormon) Dave Keuning, whose son is one year old; he couldn't countenance being on the road and not being around his own kid.

Brandon Flowers - sometime pop fantasist and wearer of eyeliner - is an old-fashioned kinda guy. He's the first to admit that he's a sentimentalist. He's becoming more comfortable talking about his faith.

'I believe in God,' he says. 'It's a big part of my life. You can bring it up and talk about it without being "Christian Rocker".'

The Killers' huge success over the past three years means he's seen a lot of the world. And of himself. Possibly too much. Now Sam's Town is the powerfully honest sound of a young man who came home, who's working through all his traumas and confusion and contradictions without recourse to abstraction or clever-clever wordplay. It's naked, and it's uplifting, and in places it's refreshingly uncool.

If you ever wondered what a cross between Morrissey and Springsteen would sound like, wait till you hear 'Read My Mind', set to be the third single from Sam's Town. It's Flowers's homage to 'the good old days, the honest men, the restless heart, the promised land, the subtle kiss that no one sees ...'

'I used a bunch of cliches that are dying and that were good to have around,' says Brandon Flowers. 'In 50 years I don't think you're gonna look back at 2006 and say "the good old days". But when you talk about the good old days [of] the Fifties, there really was something good about it. Whereas right now it's like we're creeping closer and closer to hell!'

Backstage in the Empire Ballroom, Brandon Flowers, dressed like a cowboy, ups and prepares to perform. He's been nervous about the show, at which various family members are in attendance and which has been bedevilled by technical problems all afternoon. As it transpires, it's a celebratory triumph, and onstage the joy will be writ large across his eager little face.

But right now, before showtime, Flowers is troubled. Out front, the bar girls and hostesses are preparing to meet the needs of the audience. This being Las Vegas, they are wearing little more than underwear. 'I don't know how their nipples don't fall out,' says the

Killers' frontman forlornly, and he's not too happy about it.

· Sam's Town is released on 20 October