Normally you are told to only mouth the words without sound, but when they want to record “atmosphere”, they instruct you on small dialogues.
During the filming of a movie set in the late 19th/early 20th century, we were extras for a party scene. The casting sheet even said “aristocratic guests” (yay!). This involved nice dresses with not-so-nice corsets underneath plus elaborate hair. Everyone at the party had arrived in horse-drawn carriages (filmed at some other day). We were given champagne and cognac glasses with liquids that turned out to be fruit juice.
For the “dialogue” we went through several rounds of :
“Are you having a good time?”
“Splendid, and yourself?”
“Marvelous. Everyone is so elegant here.”
“More wine?”
“Oh, yes, please.”
None of this was actually picked up by the microphones as individual words, only pleasant background noise.
Then my boyfriend did a variation:
“More wine?”
“No, thanks.”
“Oh, are you driving tonight?”
The other extra found that too funny given he was supposed to have arrived in a horse-drawn carriage and burst out in laughter.
CUT! CUT! CUT!
Party guests with director

Us, sipping on water with a tiny bit of apple juice