Primal Fear (1996)

Aaron Stampler (Edward Norton), the stuttering suspected Altar boy of the murder of the Archbishop, appears to have a split personality; when he's angry, he becomes "Roy", a dominant, more aggressive alternate person whose acts Aaron cannot remember. While changing into Roy in front of Martin Vail, he admits that he killed the Archbishop because he was the star of his home-made porn movie. That's why Aaron can't remember what happened - he was Roy at the time. Vail then tries to save his client from the gallows by claiming for his insanity in court - impossible to do as Vail has already stated that his client is not guilty. As his last hope, he unleashes District Attorney Janet Venable (Laura Linney) at Stampler, hoping she will make him mad enough to change into Roy in front of the Juries. The plan succeeds - Stampler/Roy is enraged and attacks Venable, nearly breaking her neck before he's stopped. He was diagnosed as insane and sentenced/confined to an insane asylum. Vail talks with Stampler, who can't remember a thing of what's happened. When Vail informs him that he will not be executed, Aaron is excited, thanks him and apologizes on what he did to Venable, hoping her neck will recover soon. Vail is about to leave the prison cell when suddenly he's struck by the horrible truth. "How did you know about Venable's neck?" he asks Aaron, who's supposed to forget everything... Aaron was faking all the time. He never had split personality, and he acted as a mentally Ill person in order to avoid execution. Disgraced Vail exits the prison, and walks away.