Life, eh? One minute you’re down on your luck, banging out tunes to half-filled venues in one-horse towns, the next thing you’re fielding calls from Steven Spielberg and Al Gore, getting solicitous texts from Salma Hayek, and turning down juicy roles in blockbuster movies. This is the story of 37-year-old Glen Hansard, lead singer since 1990 with the Dublin-based rock group the Frames, and songwriter star of the foot-tapping, heart-warming Sundance Festival smash Once. “This is the sort of success that I’ve been waiting for – and dreaming of – for years,” says Hansard. “But then somehow it just happened overnight, thanks to this movie.”

We meet our hero backstage at, appropriately enough, a European rock festival in the picturesque city of Ghent. Hansard and the Frames are headlining tonight’s gig. It’s sheeting summer rain outside, and so “the boys” have retired to the band’s VIP Portakabin, where Hansard sits amid whisky bottles and a fridge full of beer. His mood is ebullient as he describes his schedule – in 48 hours he’ll fly to the US to appear on Jay Leno and Carson Daly, after which he’s off to New Zealand and Australia to support his childhood hero Bob Dylan on tour. Tonight, however, the talk is not of music, but of movies, and more particularly, of Once.

“The film is about relationships, it’s about music, tension, and romance,” says Hansard, whose only other screen credit is an early Nineties turn as the guitarist Outspan Foster in The Commitments. “And it’s a film that you can only make once in your life – totally against the odds, everyone working their bo*****s off for nothing, and yet pulling out the stops at every turn.”

At its essence, Once is a quasi-autobiographical musical drama cooked up by Hansard and the film’s writer-director John Carney, a former Frames bass player. It describes the adventures of a broken-hearted busker (Hansard) who falls for a young Czech immigrant (Markéta Irglová) on the wintry streets of Dublin. The couple share a passion for music (Irglová is a real-life Czech musician), and much of the film is taken up by their softly moving folk-rock duets and their desire to cut a disc.

At times funny, at others sweetly naive, and regularly punctuated by bravura musical moments (the impromptu first-date performance of Hansard’s Falling Slowly ballad is a genuine showstopper), the film nonetheless succeeds because of the compelling and deeply ingenuous turns of the two star-crossed leads. Which is lucky, explains Hansard, because he never even planned to be in it. “I didn’t want to act in it,” he says. “I had written all the songs, and the story kind of described a life like mine – I had been a busker, I had formed a band from the streets and recorded an album. So the last thing I wanted was for the film to be perceived as some kind of vanity project.”

Instead the rising movie star Cillian Murphy (Sunshine) had been snagged for a lead, complete with generous financial backing. Early in development, however, Murphy withdrew. “I think he felt he couldn’t sing the songs,” says Hansard. “And he didn’t want to act with a non actress like Markéta. So, at that point John thought, ‘Maybe I’m better off having musicians who can half-act rather than actors who can half-sing.’ And so he decided to go with me.”

Minus Murphy’s star wattage, the film was made for a paltry £80,000, shot over 17 gruelling days and completed in time for this year’s Sundance Film Festival, where it was duly nominated for the Grand Jury Prize and the Audience Award (it won the latter). Hansard says that everything changed at Sundance. Halfway through the week, people began stopping him on the streets, inviting him to parties. Then the festival rescheduled, by popular demand, three more screenings of the film. And then the reviews poured in: “Close to perfect” said the The New York Times; “The best musical film of our generation,” said the Chicago Tribune. Hollywood studios began circling and soon the legendary mogul Harvey Weinstein paid a visit. “He said, ‘You’ve made a great date movie. The guys will like her, the girls will like him. I’d like to do [buy] it, but we’d have to change it a little.’ John was like, ‘No f***ing way!’ ” Weinstein passed and Fox stepped in.

Everything since then has, in many ways, been bathed in the glow of Tinseltown kudos. “I was with John in a café two weeks ago and Steven Spielberg phoned, just to say that he saw the film and really liked it. Then Ridley Scott rang and offered John a film about an agoraphobe to direct. I got a text from Salma Hayek, asking if I’d come and do some work with her. And Al Gore has invited us to dinner! S*** like that coming in is just frightening!”

Hansard refuses to be swayed by stardom. Later in our wet festival night, backstage in Belgium, after a roof-rattling performance from the Frames, he says that fame is a bubble, and that the real story is the rising of that bubble. “When the bubble hits the top, and pops,” he says, with an air of rock mysticism, “then that’s its death, and the story’s over.” He adds that he won’t be tempted into acting full-time and will instead stay true to his musical roots – which began on the streets as a 13-year-old busker when a frustrated headmaster offered him the immortal advice: “You can tell me the track listing of Bob Dylan’s Street Legal, but you can’t tell me the square root of nine! Go! Leave school today! Get your guitar and start at the bottom!”

After some persistent prompting, and some late-night Belgian hospitality, he admits that there are roles out there that are tempting, and with his current high profile it’s difficult to banish thoughts of acting for ever. “There was one particular offer that I got, and turned down, that was amazing. I really can’t talk about it, but it was a graphic novel adaptation, shooting in Vancouver, and I got offered the redheaded part [0.34 Google-seconds later, and the film is Watchmen, and the part is vigilante super-hero the Comedian]. I turned it down because I needed to stay out there, and keep fighting for Once, to make sure it has a life.”

Surely though, after all the media hoopla and studio interest, a sequel to Once is now inevitable? “John has made us promise that we’ll all get together in ten years’ time and do a catchup sequel,” he says. “We’ll see what the characters are doing, where they’ve got to with their lives.” And the title? “ Twice.”

Once is released on October 19

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