“Pretty Poison” (1968) is well worth seeing. I think you will get more out of it if you are familiar with the careers of the stars Anthony Perkins and Tuesday Weld. Their prior films (especially Perkins in “Psycho”) give the audience expectations about which of the two characters is really crazy. It’s a real shock when you realize the true answer.
This article is about “Pretty Poison” and “Gun Crazy”, movies with similar relationships between the main characters.