The Dark Mirror is a superior entry in the "good twin, evil twin" subgenre. Actors, of course, love the chance to play twins, for the obvious reason that doing so really gives them a chance to strut their stuff. In Mirror, Olivia de Havilland takes full advantage of the opportunity, turning in a tour-de-force performance (or perhaps it's a pair of such performances) that is a delight to behold. De Havilland was one of the finest actresses of the '40s, perfectly capable of playing both roles suffused with sweetness and light and those with a tougher, more aggressive tone to them. In Mirror she gets to do both, meeting the challenge effortlessly; she also handles the more difficult job of keeping the audience guessing as to which character is which for much of the early part of the film. Indeed, De Havilland deserves the lion's share of the credit for Mirror's success, as the screenplay has dated somewhat over the years. Its basic structure is still sound and much of the dialogue and characterization is solid, but the script's psychological underpinnings are a bit simplistic. And while the twins' habit of wearing necklaces with their respective names on them is helpful, it comes across as more than a bit bizarre. This is one of the rare slips in Robert Siodmak's otherwise assured and effective direction (another slip being Dimitri Tiomkin's occasionally over-sprightly music accompanying early parts of the investigation). The Dark Mirror also boasts a very solid supporting cast, with especially fine work from Thomas Mitchell and Lew Ayres.