Most of the press covering Once calls the film a movie musical. Writers are quick to disclaim that characters don’t burst into song the way the old movie musicals did, but important emotional points of the story are expressed in music.
It’s not an inaccurate description, but it doesn’t paint the entire picture either. Once reminded me more of the early Duran Duran videos directed by Russell Mulcahy, where the songs became miniature movies themselves. As far as my 13-year-old self was concerned, who needed Raiders of the Lost Ark when I had "Hungry Like the Wolf"?
Music video hasn’t really tapped into that kind of cinematic sense since, not after Michael Jackson turned the form into some kind of Cecil B. DeMille canvas. Once manages to integrate that early-era music video storytelling into a larger form. Like an opera or a musical, the film’s scenes build to a point where music comes in, but instead of an aria, it’s a music video.