I would have thought Véra Clouzot is younger than Simone Signoret, which made me wonder why Signoret is cast as the mistress and not vice versa; but after checking IMDb, I found I was wrong. It must be Clouzot's little fragile body which made her look younger than her actual age. Near the end, the suspense is almost unbearable, the tense is too high, that I feel my heart is as weak as the sick wife. The inspector is rather annoying, for he seems to appear anywhere, including in Christina's (the wife) room; and we see the concierge complains how people can get in and out without his knowledge.
The scariest thing for me is when Nicole (the mistress) shows Christina a mysterious face who appears in the window on the photograph. At this point, I hoped I was watching not a horror movie, but only a thriller. The ending is also eerie: the boy says Christina gives him back the slingshot... and he looks serious.