Goodfellas
An extra’s entire job is to blend into the background. Indeed, for the movie’s purposes, extras aren’t real people – they’re a set dressing that moves. Whatever they do, they should never steal the focus away from the actual stars.
It doesn’t always work that way. In Goodfellas, Ray Liotta and Robert De Niro’s characters have a tense psychological stand-off at a diner.
It’s only slightly undermined by the guy behind De Niro, who was clearly told not to look at the camera and tries to do anything in the world except that. He only succeeds some of the time.