Ava DuVernay is a film-maker. She directed Selma, which was nominated for the best-picture Oscar, in 2014 and 13th, a powerful documentary on the prison-industrial complex, two years after that. We meet as Origin, her hybrid take on Isabel Wilkerson’s book Caste: The Origins of Our Discontents (published this side of the Atlantic as Caste: The Lies That Divide Us), reaches cinemas. But DuVernay is more than that. Following the release of Selma, which deals with a key moment in Martin Luther King’s career, she found herself appointed as an unofficial ambassador for the black cinematic community. It just happened....