The Dark Mirror is an entertaining melodrama, fusing elements of film noir, psychoanalysis and an old cinematic trope about good twin vs bad twin. Although the psychology has dated badly, the film is more than lifted by clever direction, great lead performances, and some, for the time at least, astonishing special effects
Terry and Ruth Collins (both played by Olivia de Havilland) are identical twins, one kind and caring, the other a very disturbed individual. However, both are caught up in the murder of a doctor after witnesses place one of them with the victim just before his death. The police agree, but are stumped as to which, so they call in the help of psychiatrist Doctor Scott Elliott to see if he can shed any light on them. However, is Elliot putting his own life at risk, by falling in love?
Although the events in this film are triggered by crime, director Robert Siodmak favours mind over murder, with the focus on Elliot delving into the psyche of the sisters, rather than any police investigation. Olivia de Havilland wisely plays the pair as having distinctive characteristics, particularly in their attitudes to men, without being too dissimilar, or too extreme and theatrical.
Her performance is greatly helped by the extraordinary (for the time at least) optical effects used to place both twins in the same shot, which, when combined with clever editing and body doubles, means it is often possible to forget you are watching the same actress.
She gets able support from Thomas Mitchell as the baffled police detective, Lt. Stevenson, bringing the same sort of likeable enthusiasm he gave to the role of Uncle Billy in It's a Wonderful Life. Lew Ayres is less successful as Dr Elliot, although this may be down to the character being fundamentally unlikeable, a slightly pompous know-it-all, whose actions, particularly taking a patient out on a date, can seem a little unethical.
Siodmak is considered, in some circles, “the primary architect” of Film Noir, and there are enough elements of Noir, both in style and character, for it to be counted in that genre. The film is shot with plenty of stylised light and shade, which helps keep an air of suspense and unease, the plot revolves around crime, and, of course, if there is a woman who is aggressive confident, and intelligent, she has to be evil. Siodmak has also taken the time to put plenty of mirror themes into the imagery and characters, such as the symmetrical Rorschach inkblots in the opening and end credits, or the total opposite character pairing of the old-fashioned cop and the ultra-modern doctor.
What also makes this film so fascinating is the portrayal of psychoanalysis, something Hitchcock had started putting into his films with Spellbound, which was released a year before this. Perhaps it was the hope it seemed to offer in healing the minds shattered by the traumas of World War Two, but it is shown as something that can provide, quantifiable, empirical, definitive answers, and quickly, in the same way a treatment of drugs can for the body. At one point Elliot subjects the twins to a Rorschach inkblot test, showing them a series of pictures and noting their answers. He is then seen crosschecking these against some sort of textbook, before going on to tell Stevenson that from the results of this alone can he tell for certain that one of the twins is insane. This kind of thinking looks clunky and simplistic nowadays, although this film was not the only example. The idea that a psychological test or the uncovering of a single traumatic event can provide the key to unlocking a mystery would crop up in other Film Noir movies, throughout Hitchcock’s work, and in the giallo films that came out of Europe in the 1960s and 70s. This suggests a pattern of thinking in directors (maybe also in audiences) that, no matter how baffling a mystery may be, it cannot, under any circumstances, go unsolved.