Movies can be viewed as a form of hypnosis. Your mind is taken out of the real world and your conscious awareness is taken into a fictional world where you, the personality you are in real life, is no longer present. They are written and designed to cause you to associate with the protagonist or the main character for the duration of the film for emotional impact. This is fine for most people without DIDs, but it can mess with those minds who have the disorders.
It's the same thing with certain video games.