Joining a rock band. Sometimes it means working with a brilliant, though mentally ill, character who never takes off a big fake head that looks like a combed Bob’s Big Boy. There’s also that guy’s violent and impulsive girlfriend, as well as a suicidal burnout and some French artsy-fartsies. The big subject of this comedy is creativity. It does its awkward best to render the act of songwriting as cinema and it offers a few retorts to the naive idea that genius and insanity go hand-in-hand. Here, we get one hack songwriter, though a nice guy (Domhnall Gleason), and one effortlessly musical nutbag (Michael Fassbender, masked throughout the movie). The hack’s in awe of the fruitcake’s talent and starts to credit the guy’s skill to his cloudy mental state. The film argues him out of this, as well as a few other bad ideas he’s got. We’re left with a movie that’s not perfect, but that should go down as a classic of the noisy indie rock aesthetic shaped up and shined up (but not too much) for the screen. It embraces a few feel-good moments and smartly avoids others. Drama invades the comedy and happiness horns in on the sadness like bursts of guitar feedback.