Joe Otterson TV ReporterSophie Turner has joined the cast of the HBO Max series “The Staircase.”Turner joins previously announced cast members Colin Firth, Toni Collette, Parker Posey, Juliette Binoche, and Rosemarie DeWitt.The eight-episode series is based on the docuseries of the same name as well as various books and reports about the case of Michael Peterson (Firth), who was accused of murdering his wife, Kathleen (Collette), in 2001.