There's a moment early on this film that nicely summarizes Damien Chazelle's approach to music and art: Ryan Gosling, sitting at his piano, puts on a jazz record, which begins with a complicated piano riff. He stops the record, plays the riff on his piano exactly as heard on the record, messes up, then spins back the record, replays the opening, and mimics it again. It's the same methodology behind the "Play one wrong note and you DIE" premise of…