William Finley (Bobbi) is a regular in writer and director Brian De Palma's movies. He also appeared in Sisters a few years before. He was known for being an odd villainous looking character with a haunting voice. DePalma uses only his voice in this movie.
The extended tracking shot in Danielle's apartment following the murder of Phillip was influenced by Max Ophüls and directly references Alfred Hitchcock's Rope.
Brutal murder with knife. The tracking shot of Jennifer Salt walking up to the experimental hospital was influenced by the tracking shot of Martin Balsam walking up to the Bates house in Alfred Hitchcock's Psycho.