Les Diaboliques

Les Diaboliques (also known as Diabolique) is a French suspense-thriller, made in 1955 and directed by Henri-Georges Clouzot. The wife and mistress of a Jerkass school headmaster conspire to kill him, but after they carry it out, his body disappears, and then things just get weirder. The two main ladies were played by Vera Clouzot and Simone Signoret.

Remade as Diabolique in 1996 with Sharon Stone, Isabelle Adjani, Chazz Palminteri, and Kathy Bates.

Tropes used in Les Diaboliques include:

Do Not Spoil This Ending: Translated from French, the final title card after the film reads:

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.

Faking the Dead
Focus Group Ending: The remake.
Follow the Leader: Deathtrap, with gay conspirators.
Gaslighting
Gender Flip: Kathy Bates as the Fichet-Expy in the 1996 remake.
Hollywood Heart Attack: Averted. The fatal heart attack at the end is disturbingly realistic.
Les Yay: Hinted at in the 1955 film, explicit in the 1996 film.
Mind Screw
Nothing Is Scarier
Not Quite Dead: Possibly. The film hints that the wife might have survived her heart attack. Or it was her ghost. Or the kid is a liar.
Obfuscating Stupidity: Fichet, the detective, employs a bit of this. (More than one observer has noted the character's similarity to Columbo.)
Reverse Whodunnit
Spooky Photographs
Twist Ending