Jean-Pierre Melville's Les Enfants Terribles (1950)

Les Enfants Terribles may be a great film because of Cocteau's novel, but it is that, especially in spite of, and beyond, the novel.
---Noel Burch

Other devices -- such as the unusual camera angles, rhythmic editing and expressionist lighting -- elevate Melville's dual strategy (with the help of Decae), to transform a theatrical space into a cinematic one, while maintaining the "theatricality" of Cocteau's universe.
--Ginette Vincendeau

I would have liked a more fragile. more tender actor in the role of Paul, more ambiguous even.  Edouard Dhermitte, with his great strength, left no room for ambiguity.
--Jean-Pierre Melville.

Les Enfants Terribles is an impressively low-key film about youth, incest and gender fluidity.  Melville, who was chosen by Cocteau to direct the film after Cocteau had seen Melville's first film Le Silence de la Mer (1947), brought his intelligent style to following the novel relatively closely, with the help of cinematographer Henri Decae, the music of Vivaldi and Bach and an elliptical narration by Cocteau himself.

A brother and sister, Elisabeth and Paul (Edward Dhermitte and Nicole Stephane) live together in a cramped apartment and see little of the outside world, except for Dargelos and Agathe (both played by Renee Cosima), as their relationship gradually become more physically incestuous.  Eventually Paul and Agathe confess separately their love for each other to Elisabeth, who keeps it to herself and convinces Agathe to marry their friend Gerard instead.  This leads to Paul killing himself with poison and Elisabeth then killing herself with a pistol.

This was only Melville's second film and, understandably influenced by Cocteau, it contains a bit too much surrealism --dream sequences, a statue with a drawn-on mustache, Cocteau's rather obscure narration, etc. --for my taste but shows an impressive imagination on the part of Melville (very much influenced by Orson Welles at this point)  and some complicated special effects, doing a great deal with an extremely limited budget.