Wouldn't it be convenient, if you were arrested for murder, to be able to say, "It wasn't me; I have a twin!"? In The Dark Mirror, witnesses see Olivia de Havilland fleeing the scene of the crime, but when it turns out she has a twin, she can't really be identified. Both twins have alibis, so neither one can be arrested. In comes Lew Ayres, a psychiatrist anxious to study the pair of twins. In an incredible example of trick photography (which I still can't figure out to this day), they talk to each other in the same frame, and even touch hands! Olivia's "split personality" is very effective; throughout the movie, I kept thinking, "Is that Olivia or the other one?"
Some might find this movie predictable, since it has an "evil twin" plotline, but I liked it. It was exciting, and Olivia got to have a lot of fun acting as two different ladies. If you like this one, check out Bette Davis's turn as twins in A Stolen Life.
Review of The Dark Mirror
