The first time we see 15-year-old Minnie Goetze in Marielle Heller’s extraordinary and often extraordinarily uncomfortable “The Diary of a Teenage Girl,” the aspiring cartoonist is walking — no, striding — down a sun-dappled San Francisco street. It is 1976, and our heroine looks like she could take on the world. Then, her voice-over: “I had sex today. Holy (expletive)!”
Played with Oscar-worthy emotional precision by 23-year-old British actress Bel Powley, Minnie breaks out a knowing, almost gleeful smile; moments later, we see her toggle between confidence and insecurity. It’s a bravura opening that signals the complexity to come.