The Dark Mirror relies on some atrocious psychology to power its premise, but if you can rightly acknowledge that and still sit with the film, you’ll enjoy a fiendishly fun double role for Olivia de Havilland. She plays both Ruth and Terry Collins, twin sisters who become suspected of murder when a doctor they knew ends up with a knife in his back. The investigation is complicated by the fact that often—such as a job at a candy counter—the sisters don’t tell people they’re twins, so that they can split the workload. When not fooling people, Terry and Ruth wear necklaces with their names on them. The distinguishing touch isn’t necessary for us as viewers; de Havilland has differing mannerisms—from her posture to the wideness of her eyes—that allow us to tell the two apart. It’s an impressive demonstration of her range, given Terry and Ruth’s distinct personalities. But beyond that—especially with Terry, who is the more assertive of the sisters—she delivers an intensely compelling, deliciously duplicitous character. As the movie darkens (director Robert Siodmak and cinematographer Milton Krasner gradually gather noir shadows), de Havilland provides enough (double) layers to keep us wondering: do Terry and Ruth use being twins as a way to deflect the sexism they regularly face (it sure helps foil the leering cops) or do they employ their biological uniqueness to more nefarious ends? The Dark Mirror keeps you guessing until the very end.