Glen Hansard is the leader of a hugely influential rock band in Ireland called The Frames. They are one of the great bands of the past couple of decades, but for some unknown reason never made it big in the US. John Carney began as the bass player for The Frames in the early 90's but slowly gravitated away into filmmaking. But when he got an idea to do a movie told mostly through music, he wanted his old bandmate to write the music for it. Irish star Cillian Murphy (he was Dr. Crane/The Scarecrow in Batman Begins) was supposed to play the lead, he was a musician prior to becoming an actor, but walked away after hearing the proposed music that he wasn't sure he could pull off. Losing a star meant that Carney wouldn't be able to get the kind of funding he would've previously been able to get. Instead of dwelling on this, Carney turned it into a positive. This meant that he could make his movie for basically no money, which meant no interference from a studio. He also re-evaluated his leading man, thinking it might be better to get a great singer who could half act, rather than a great actor who could half sing. He turned to Glen Hansard, who promptly refused, after having had bad memories of his only previous foray into acting, in Alan Parker's The Commitments. But, thankfully, Glen reconsidered and agreed to do the movie. Playing the female lead opposite Glen would be his good friend, and recent musical collaborator, Marketa Irglova, a young Czech pianist/singer whom Glen had met a few years previously. Marketa added her haunting voice and piano to Glen's songs and they became the backbone of Once.
The movie was ultimately made for very little money (just over $100,000) and looks like it. The visual quality of the film makes it clear that it was shot on cheap cameras and shot very quickly. This would be a hindrance to some movies, but actually works here. Glen's character (unnamed, but credited as The Guy) lives in a working class section of Dublin, splitting his time between working in his dad's vacuum repair shop and being a street musician. Marketa's character (The Girl, or "herself" as Glen tends to refer to her) splits her time between being a maid, and being a street vendor (roses, magazines, whatever). They meet one night as Glen is playing his own heartbreaking songs to no audience, he tells her he plays his songs at night because people only want to hear things they already know during the day, and that's how he makes money. She likes his songs, and Glen soon finds out that she plays piano. She takes him to a music shop where the owner lets her practice for an hour every day at lunch (she doesn't have a piano at home because they're too expensive). Before long they're collaborating on music, and eventually recording in a small studio.
As a musician every one of the musical sequences rings true, probably because the movie is about musicians, so it's not like the characters are just randomly breaking out into song. They're simply playing their songs the way that musicians do. There's the possibility that their relationship could lead to a romance but, despite the embarrassingly photoshopped poster showing them holding hands, it doesn't. And the movie is better off for it, because that would've been the first demand from a studio, the guy and the girl must fall in love. Well, actually they still do, but not in a traditional movie sense. There's no doubt in my mind that these characters love each other possibly more than they'll ever love anyone else, but they don't act on it. Not that they don't want to (her million-watt smile and his puppy dog eyes tell a great untold story), but it just isn't in the cards for them.