Set in a mid-70s San Francisco, the new film Diary of a Teenage Girl is an unflinching and deeply sincere coming-of-age story about a 15-year-old's sexual awakening and secret relationship with her mother's boyfriend.VICE sat with the movie's breakout star, Bel Powley, to discuss the several challenges of her performance, working with Kristen Wiig and Alexander Skarsgård, and taboo subject of female sexuality in films.Diary of a Teenage Girl is in cinemas now.