Like laughing about the setup of a scene that is about to unfold.
Or getting frustrated about a movie breaking (or lacking) consistency in the underlying mechanics of the world its drawing.
I'm constantly in this situation, starting to laugh about something that is building up below the surface of a scene, just to frame something (entirely else) that is then presented directly to the viewer in that scene. While people sitting next to me look bewildered at my face and back at the screen...