If you've seen Once, you probably remember this pivotal scene. Guy teaches Girl "Falling Slowly" with a quick run-through of the chords and changes, along with the lyrics. They perform the song quite well, and for me, quite convincingly. Yet there is a degree of "come on, they can't pull off a song like that with such little rehearsal!" Well, though the actors were better at playing music than acting, I totally believe it because I've experienced that kind of fresh, exciting spontaneity plenty of times before while playing in bands.
I can firmly recall running through a song with my high school band only once and recording the next run-through. Maybe it was our ability to read each other's body language, or just understanding the simplicity of repeating a riff four times before a change, but we came up with something cool without much tinkering. Somehow we knew where the dynamics were and how long to stretch everything out. That's one of the really mesmerizing things about playing music with people.
Yet I'm reminded of when I saw this kind of spontaneous collaboration in other movies, as well as a certain play. Frankly, it seemed really fake and non-believable. I saw The Buddy Holly Story performed in London, and a rather memorable scene involved the recording of "Everyday." The deal was, the keyboard solo that seemed to be made up on the spot didn't appear to made up on the spot. It seemed rehearsed -- too rehearsed.
The same went for a scene in Oliver Stone's The Doors, where Ray Manzarek (played by the one and only Kyle MacLachlan) fumbled around for a keyboard part for what became "Light My Fire." Somehow and somewhere between the fumbling around the notes with no time signature and coming up with the signature line in perfect time, it seemed to happen instantly. Not to me.
A big, fun part about playing music is the sense of collaboration. But being in situations where people are told exactly what to play (and how to play) by one person are not "band" situations per se. Not everybody's heads are filled with streams of melodies, beats and harmonies, knowing exactly what they want fleshed out on record. The magical surprises that come from someone else's input can render something you never imagined. So seeing that captured in a film like Once, I add that to one of the many other things the filmmakers and actors got right in making that movie.