Like the preceding comment by JimShine illustrates, this movie is only half-good : an average-to-good acting job; and indeed the direction lacks energy and tension.
The strength that remains in this movie comes basically from its clever story, its suspenseful plot. The explanation is simple : it is adapted from a novel by Boileau-Narcejac! Voilà ! Of course ! For the sake of anecdote, let's remind the reader that the British publishers of Faces in the Dark / Les Visages de l'ombre (that was in the early 60s) asked Pierre Boileau and Thomas Narcejac if they couldn't, by any chance, change the conclusive chapter because it was too dark. Well, if they called it 'Faces in the Dark', it must have been for some reason, bozo ! Like many people in the same trade, Boileau-Narcejac wrote and published too much. As a result, let's say MOST of their novels are at least enjoyable, SOME are pretty boring - especially the late ones, as often happens with thriller/suspense writers - and a solid DOZEN are masterpieces of suspense, atmospheric and chilling to the bone ! They also wrote an adorable jewel of the pastiche, 'Arsène Lupin : Le Secret d'Eunerville' which is a full-fledged Arsène Lupin novel better than some of the original ones written by Maurice Leblanc.
In fact, lots of people may know very well a couple of their stories without realizing it : H.G. Clouzot's Les Diaboliques and Alfred Hitchock's Vertigo were both adapted from a Boileau-Narcejac novel.
They also can be counted are among the rare true INVENTORS of the genre, because in the immediate postwar years, they decided to escape from both the whodunit (written from the point of view of the detective) and the noir (written from the point of view of the criminal) and decided instead to write their novels - a genuine breakthrough - from the point of view of the victim. And it is the case with Faces in the Dark, which could well be their most efficient suspense ever - but to know that, you have to read the book, for this movie adaptation, though acceptable, doesn't really do it justice. Reading 'Les Visages de l'ombre' is a really frightening experience (especially the conclusive chapter!), while watching this movie is at best intriguing.