The advance hype is attempting to sell Once, a musical about a Dublin street performer, as “inspirational,” calling up unspeakable images of an in-the-round production of Godspell, complete with clown-faced actors.

But Once is neither sermon nor saccharine. It is, however, intelligent, low-budget and irony-free.

Glen Hansard, lead singer of the Irish rock group The Frames, plays one half of the film’s romantic duo – “The Guy,” the only name the character is given. The Guy is a passionate “busker,” (slang for street musician), who lacks the courage and confidence to pursue a singing career, and instead holds a job repairing vacuum cleaners for his dad.

Marketa Irglova, also a musician, plays “The Girl,” a young immigrant making her way through a strange town, who stops, listens and likes what she hears. Since she also has a Hoover on the fritz, the pair spend a week together exploring their talents and convincing one another to see what each had only dreamt about before. (OK, so it’s a little inspirational.)

While some have criticized director John Carney’s lack of character names as being “affected,” Carney says the choice was intended to achieve an objective effect. “I called them ‘guy’ and ‘girl’ for the first 20 pages like every writer does ’cause he hasn’t thought of a name. Then I started thinking on names by looking in the phone book. I thought, let’s just keep these characters at a distance and observe them like you do people on the street. It’s like looking at really interesting strangers.”

This free-form process extended to the creation of the entire movie, Carney says. “Everything was written out ahead of time and the three of us would improvise. The whole point to this film is naturalism.” Even the film’s title is impromptu: “I kept saying, ‘Once I get my friends together we’ll start a band. Once I start the band we’ll cut a record. Once I…’”

It’s how life really works, and Once may be the closest cinema is likely to come to a neorealist musical.

Shot on location, without permission and (with a nod to B-movie economy master Roger Corman), in a mere 17 days, Once tells a very modern, very simple love story with just a few songs “smuggled” in. “The characters are musicians so you don’t have to step outside the film to suspend your disbelief,” Carney says. “It was a question of trying to stage a number of set pieces which were actually musical numbers threaded into the dialogue. This is not to say that there isn’t a three-act structure in the story; it’s just a little more oblique than in your average film.”

Still, the cinematic frame of reference for Once is fairly straightforward – “a lot of stuff on YouTube” and certain rock videos, Carney says. “I also knew it was probably going to be a ‘two-hander,’ so I thought of Before Sunrise and Brief Encounter – films I like with two people.”

Carney had originally called upon Hansard to provide anecdotal material and songs but not to act.

“I was just delighted to be a part of it,” Hansard says in his gentle Irish brogue. “For a guy in a band to get a few of his songs in a movie is a great idea. My job was the music guy, like Tom Waits on the set of Coppola’s One from the Heart.”

Carney didn’t immediately cast newcomer Irglova as “The Girl,” either. The actress he’d originally cast, Cilliam Murphy (Breakfast on Pluto, Batman Begins), who would have brought a certain amount of name recognition, pulled out of the role causing the project to be temporarily shelved.

When Murphy backed out, Hansard stepped in. He’d done some acting but it hadn’t been a positive experience. “When I did The Commitments,” he recalls, “[it] was really a pain in the ass. I was just starting my musical career and the first seven years were completely overshadowed by two months shooting I did on someone else’s art.”

But he says he thought he couldn’t lose with Once. “Best case scenario is, for months people are going to connect the dots between The Frames and Glen. If I’m a shit actor it really doesn’t really matter. What’s important for me personally is that the songs will get out there.”

He also thought that Irglova, with whom he’d been making music, would be perfect for the role of “The Girl.” Although the character was initially written for a woman in her 30s and Irglova is much younger and had no previous acting experience, “I asked [Carney] to meet her [because] she plays piano [and] is Eastern European,” Hansard says.

“It makes more sense to have two musicians that can half act rather than two actors that can half sing,” Carney says of his decision to cast the pair. It was the music that needed to be convincing, he realized; the acting could be worked on.

The music, it seems, comes naturally to Irglova. Unlike her assertive on-screen alter ego, she sits quietly during our interview, but at a post-show Q&A at Landmark’s La Jolla Village she slings her guitar strap over her shoulder and sings song after song.

All three are singing a new tune now that Once has won the World Cinema Audience Award at Sundance. Were it not for that distinction, “This film would quite literally be on the shelf,” Carney says matter-of-factly. “You wouldn’t get people to see it. They’d watch it on DVD and say that it was quaint and nice…. But it wouldn’t have a life.”

The award even sparked a bidding war – “a small one,” Carney coughs modestly, “but a realistic one. The award helped [potential backers] see the film in a different way.”

Hansard is still wide-eyed with astonishment about it. “When the audience at Sundance responded, it was like we got away with it. Fucking, hell, the reviews have been amazing. It comes to a point where you say, ‘What did we do?’”

It’s a once in a lifetime kind of feeling: for Once, a good musical.