Superb tension-relief joke in MM (spoilers galore)
You all know that Truffaut studied Hitchcock's narrative devices, especially the transference of audience identification (a famous case: when you want Marion Crane's car to sink properly in the bog, whereas 10 minutes ago, you were revolted by Marion's murder), and also the tension-relief joke (good example, the elevator scene in North by Northwest where Jessye Royce Landis asks the killers "You gentlemen aren't really trying to kill my son, are you?")
In Mermaid, there is a wonderful short scene that blends both. The fake Julie's canary is dead in its cage. Louis, afraid to break her heart, goes slowly to her door. She's in the bath. Through the door, he tells her "I have sad news, your canary's dead." And she replies casually "Oh yeah, I'm not surprised, he didn't look so hot lately." It is all very interestingly constructed as a suspense scene. You want Louis to find out Julie is a fake, yet you also share his reluctance to upset such a beautiful woman. You expect her to overreact, and her off-hand answer is a complete shock. This short scene is a gem. Truffaut's Hitchcockian's scenes can be awkward and clumsy (often because his actors are less malleable than Hitch's), but this canary scene is perfect.
And then, if you watch Mermaid twice, there is the suspicion that devilish Marion, the fake Julie, may have throttled the canary as well...