At the beginning of the movie, the brain of Dr. Edwardes is a mystery. What's in that brain anyway?
Not only don't you know, but "Dr. Edwardes" doesn't know either. On the outside, that doctor looks smooth and handsome—like Gregory Peck, in fact. But behind the eyes and between the ears, it's all shrouded in mystery and weird curtains made of eyes.
Psychological thrillers usually focus on the weird brains and neuroses of the main characters. Mystery stories are usually about… well, mysteries. In psychological thrillers, you're asking what makes this confused person tick? In mysteries, you're asking who stabbed this person with the candlestick behind the salad bar?
Spellbound, though, is tricky, because it turns the two questions into one. Constance and Brulov are detectives—but they get all their clues from Edwardes'/Ballantyne's brain. It's like Sherlock Holmes unscrewed the top of the head of a witness and then dusted the brain for fingerprints. (Though, okay, that would be kind of disgusting. Act like Ballantyne, and forget we said that.)
Of course, in the real world, Constance and Brulov couldn't analyze Ballantyne's dream for clues to the murder. Analysis doesn't work like that; dreams aren't coded messages to the police. As a psychological thriller, the film's psychology is amateur-hour. As a mystery, it's contrived.