Musicians are natural-born documentary subjects: They say outrageous things, they look fabulous doing it, and they might just let you license their music for free (if they like you). Anyone who rises to rock star level fame is either a tortured soul or a creative genius — or, sometimes, both. The challenge in making a music documentary is to rise above mere hagiography and tell a story most fans have never heard before, which can be tricky when you’re dealing with people as obsessively beloved as Kurt Cobain or Amy Winehouse. In some cases, the best stories are discovered behind the scenes: the forgotten backup singers, the recluse who discovers his fame decades later, the brother living in the shadows of the rock star. Whether famous or unknown, there is nothing quite as daring as getting on a stage and singing your guts out — and no creative skill as revered as writing a song that moves people.