The preferred term is "Background artist".
If they speak they get a higher fee, so are instructed not to speak, apart from which, it can play havoc with recording. Scenes are filmed from different angles one at at time, so any noise in the background would chop and change with each different angle chosen for the edit.
The director doesn't instruct the background artists, again, they would get a higher fee for being instructed by the director, it is the assistant (or 2nd or 3rd assistant) director that instructs the background artist.
And as EDDIE51 says, many background artists are quite happy to do their bit and not be an actor front of stage.