Two quotes on this sixty-something year old movie:
1. After it's done, we see this:
"DON'T BE DEVILS! Don't ruin the interest your friends could take in this film. Don't tell them what you saw. Thank you, for them."
So when Hitchcock said on his 1960 movie Psycho (made 5 years later): "Do not expect to be admitted into the theatre after the start of each performance of the picture ...", this is where he got that idea.
2. From Roger Ebert:
A man wrote to Alfred Hitchcock: "Sir, After seeing Diabolique, my daughter was afraid to take a bath. Now she has seen your 'Psycho' and is afraid to take a shower. What should I do with her?" Hitchcock replied: "Send her to the dry cleaners."
This ancient movie had an ending 5-minute sequence that truly frightened me. And that horror was about a dead body in a bathtub. So the French Hitchcockian film predated Hitchcock's most commemorated shower murder scene, and had repercussions that affected both Stephen King and Stanley Kubrick in writing and directing The Shining's scariest moment.
I'm scoring this on a matter of principle.