The Staircase.As reported by Deadline, the Game of Thrones actress joins Colin Firth, Toni Collette, Parker Posey, Juliette Binoche, and Rosemarie DeWitt in the HBO Max series.The eight-episode series is based on a docuseries of the same name. It will focus on the case of Michael Peterson played by Firth, who was accused of murdering his wife Kathleen (Collette) in 2001.