You see movie stars walking through a crowd and wonder why no one is staring at them. That's because this is a movie and they are extras, paid to make a background look as normal as possible without drawing attention away from the lead actors. It's one of the lowest-paid jobs in acting outside of internships, but if you are nondescript and flexible, and live near where the jobs are, you can make a living out of being an extra. There's more involved than just being yourself in the background. For example, sound must be kept to a minimum so that the dialogue can be recorded. Ambient sounds are added in later, so extras must be quiet, even when they're talking to each other in a "natural" setting.
This is generally one of the more difficult things for extras to get good at as many people find it unnatural to make silent vocalisations without overcompensating by moving their eyebrows too much or otherwise trying to use body language to make up for the fact that they aren’t making any noise when fake talking. To avoid this, beyond being conscientious of body movements, some extras practice nonsense phrases they can repeatedly mouth to one another. For a little fun, pay attention to talking extras in the background next time you watch a movie. You can almost always spot the newbies from the veteran pro extras because of things like this.
On that note, because most sounds are added in post-production, scenes involving parties, dancing, or cheering crowds are an especially surreal experience for extras, as the set is almost always completely silent while filming, other than the talent doing their thing. This poses another unique challenge for extras who are required to dance to a song they can’t hear, in time with other people listening to the same, non-existent song, all while pretending to ignore whatever the main actors are doing. And with cheering crowds, extras must be extremely careful not to actually make any clapping noises while they clap and otherwise raucously cheer silently.
It's not all silent, though. Sometimes extras are sequestered away from the action while they wait for their scene, so they can at least talk to each other. And the food is usually really good. Read about the work of an extra, and how to get those jobs, at Today I Found Out.
(Image credit: Beat Albrecht)