I’ve been a movie extra.
For long shots, street scenes with people passing by and the like, we’d generally be talking about anything: the weather, sports, you name it.
Closer shots might be the same, but they might also require silence from us extras while the principals had dialogue. So we’d talk to each other silently–mouth moving as if we were actually speaking words, but not using the voice. Try saying something like, “Did you see the Patriots game last Sunday? Brady threw some great passes” without using your voice to get an idea of what we were doing.
In one scene, another extra and I were in a bar. We were supposed to be quietly laughing and talking with a principal actor, who would spot his friend across the room, say goodbye to us, and go to join his friend. Up to that point, there would be background bar noise; after the two principals met, their dialogue would take over.
But we two extras keyed in on the “laughing” part of the director’s instruction, and told jokes to each other, very quietly. Problem was, we got the principal laughing so hard, the shoot had to be delayed a few minutes while he recomposed himself. After that, the director told us to concentrate on sports and weather while smiling a lot. We did, and the scene still worked.