The movie itself is a good example of a film that's very entertaining as long as you do your damnedest not to think about the story. And I mean you have to try really hard not to think about the story, because there's something implausible or absurd at every turn. The idea of the film (based on a Patrick Hamilton novel but with the time period changed to the turn of the century, the better to re-use stuff from Brahm's previous film, The Lodger) is that Cregar is a promising young composer/pianist with a split personality; when he's stressed out and hears loud horrible noises, it causes him to black out and commit acts of violence without being fully aware of what he's doing. At the beginning of the film, we see him kill someone; when he comes to, he knows that a) He was in that area; b) Someone was killed in that area; c) There's blood on his coat. Police shrink George Sanders investigates and concludes that there is no evidence at the crime scene that could link Cregar to the murder (huh?) so Cregar puts the whole thing out of his mind even after finding the knife with which the murder was committed (why didn't Sanders find it?).
Then later in the film, when Cregar's pal Faye Marlowe is attacked from behind after an argument with Cregar, no one suspects him of having done it -- not even Marlowe, who knows about his problems. How did he get into her house and try to strangle her without anyone, including her, having any idea he was there? Why is it that the police think that a later murder couldn't have been done by Cregar, even though he's the only possible suspect, because "he doesn't act like a guilty man?" Why does Sanders take the whole movie to figure out what he could have figured out in the first twenty minutes?
None of that really matters much, because the movie has a) A good cast, including Cregar, Sanders and Linda Darnell playing another one of the slutty characters she mostly played at Fox (I guess she got the bad-girl parts while Gene Tierney got the good-girl parts); b) Some amazing camerawork from Brahm, not a great director but a very fine visual stylist; c) A score by Bernard Herrmann. So forget the plot and enjoy those elements.
In many ways this is really Herrmann's movie; as a film about a composer who writes in several different genres -- he's writing a concerto and he also writes trashy popular songs for Darnell -- it wouldn't work unless the music was good enough to justify the idea that Cregar is talented. With an efficient but less talented composer, like Fox's head of music Alfred Newman, the movie wouldn't have worked; Herrmann's big Concerto Macabre convinces you that Cregar is a good composer and works perfectly with the cutting and flashbacks and provides a psychological portrait of Cregar's character.
Note: A little bit has been cut out of the clip below; the movie itself is complete on the DVD (if rather beat-up looking in spots).