Most of the moment, they are pantomiming, which implies they are mouthing words while matching music with physical movement rather than speaking. And nine times out of ten, it’s random, unplanned babble. When I initially went to Los Angeles years ago, I worked as an extra. I worked on a variety of projects, including Traffic, Moonlight Mile, Bedazzled, and others.
Most of the time, they are pantomiming, which means infants are mouthing words while matching them with physical movement rather than speaking. And nine times out of ten, it’s random, unplanned babble. They are frequently mouthing common conversational words.
When the director begins filming a scene, the only persons who speak are the principal actors who have speech as well as any ancillary characters that interact with them. To avoid interfering with the sound recording of the principles on set, extras are instructed to pantomime.
There are exceptions to this rule. Robert Altman was known for having many lines of conversation running throughout a scene while the camera/mic moved about the room.
But, for the most part, it’s just pantomiming. It even extends to non-verbal communication. Have you ever seen a movie scenario set at a dance or something similar? Those extras in the backdrop are almost always dancing to no music.
But, contrary to popular belief, providing a credible supplementary performance is not as simple as it appears. A former extra like myself can always recognise bad extra acting since the extras are constantly nodding yes, lifting their eyebrows and/or making large eyes in doing so, and so on. This is the worst nightmare of an assistant director (they usually direct the extras). It’s natural for extras to nod their heads when they aren’t speaking as well as feel a need to communicate in some way. The constant nodding, large eyes, massively exaggerated motions, and other forms of awful extra acting are common. I grin every time I see that in a movie.