It was doomed from the start. Irish writer/director John Carney's third film, titled Once , that is.

Carney had a budget of $50,000 (The film was completed in 17 days for $150,000, most of which was paid for by the Irish Film Board.). His big-name actors dropped out just prior to shooting (Cillian Murphy was reportedly initially snagged for the leading role, known simply as “Guy.”).

Carney replaced them with a couple of friends, one with minimal acting experience, the other with none whatsoever. To make matters worse, the film was a musical, and it was decided that Carney's audience would have to sit through full performances of eight unknown songs. Yes, in their entirety.

The film was promptly rejected at the Toronto, Edinburgh and Telluride Film Festivals. Then why is it the best goddamn movie all year?

The deeply affecting music, the authentic heart of the story and John Nein, the Sundance programmer who caught the film at Ireland's Galway Festival (one of the two into which it was admitted) and fell in love.

Carney, a former bassist for Irish indie-rockers the Frames, always loved musicals. He just hated cliché, contrivance. He wondered, could you center a movie's story and scenes around songs without everyone – the jury, the florist, the banker, the baker – breaking out in them?

“The classic musicals were made in that [symbiotic] way,” Carney explains. “MGM would go, ‘We own nine of Gershwin's songs, what are we going to do? Can you write a story around them?' They'd stitch together a story, and that's why they're so bloody good.”

It all starts with the music. Carney wanted to make a “modern-day musical,” a film whose story would organically grow from the music, from the center of the songs. The characters and scenes would tell a story, but, moreover, extend it, fleshing out the passion and emotion already inherent in the songs.

That's why Carney recruited his one-time collaborator, the Frames frontman, Glen Hansard, to work with him on Once from the get-go. And so, he and Hansard exchanged lyrics and dialogue, back and forth, for the story Carney wanted to tell: a guy – a Dublin street busker like Hansard used to be – meets an Eastern-European immigrant girl with a difficult present but a musical past (immediately cast with Marketa Irglová, a 17-year-old Czech singer/pianist Hansard met and quickly worked with on last spring's gorgeous album, “ The Swell Season ”).

The Guy, heartbroken over a cheating ex-girlfriend, finds a breath of fresh air in collaborating with the Girl, who, in turn, recalls a long-lost joy from recording with the Guy, temporarily escaping her immigrant life of single motherhood and financial struggle.

All told, a simple story where music-lovers and music-makers reach a higher, clearer sense of themselves as people through music. Carney's film is a heartbreaking, joyous, goose-pimpling success.

Thank God Murphy (or whoever else it was) dropped out, giving Carney the light bulb switch idea of casting Hansard in the leading role.

“It suddenly dawned on me that it's going to be hard for even a good singing actor to do quite what Glen [was] doing,” Carney says. “There's nobody who's going to be able to sell Glen's songs the way he does in the performance, so it became very clear to me, let's just strip it down really simple.”

The result is one of the most authentic performances – and, with Irglová, exchanges – I've ever seen on film. Carney credits John Cassavetes with his desire to be a filmmaker, and the icon's stripped-down, bare-boned aesthetic pervades Once .

“On the first day of filming, I thought it was going to be a disaster,” Carney confesses. “They were nervous, the camera was right up their noses. They were self-conscious. I realized it wasn't going to work with the camera in their face. I needed to be on the other side of the street and to use long lenses. There would be no clapperboards and no shouts of ‘action.'”

As a result, the characters walk through Dublin's trendy Grafton Street through swarms of shoppers and workers on break. The production, the uncontrived performances and the real-life characters all help to weave an authenticity that is neither burdensome nor dull, but so very refreshing.

It doesn't hurt that the music will send chills down your spine.

Once's unconventional scenes and heartfelt, true-to-life moments place it squarely in the tradition of Annie Hall and Before Sunrise / Before Sunset , haunting you long after the film is over. Its use of music is original and authentic, and you'll be humming the gorgeous melodies for days.

Once is currently in select theaters.