Movie extras are also known as background artists; the roles are strictly non-speaking and are meant to blend in. Extras don't need any acting experience and can be people of all backgrounds, ethnicities and ages. Extras are required to be able to take direction, be reliable and have prompt timing.
Same agencies specialize in only hiring extras. Sometimes directors will look for a specific look, depending on where the scene takes place. Sometimes basic skills are required, such as dancing. It may even be required of the extra to bring her own wardrobe, and some professional extras have various costumes that might make them more enticing to the casting directors.
Extras can be hired for just a day or perhaps a longer period of time. In the United States, extras don't have to be members of the Screen Actors Guild (SAG). Members of SAG are usually those who have speaking roles. However, extras are protected to receive the same working conditions as actors with more substantive roles. Extras who are members of SAG get paid on a union scale, with extra compensation for overtime, costumes or special duties. Extras who aren't members of SAG can be hired after a certain number of SAG extras have been hired. Generally, non-union extras are paid less than SAG extras are.
The life of an extra generally includes long times of waiting around. There is always a chance that the extra might not even be used on that particular day. Sometimes, the only thing an extra might do is walk across the set a few times and then wait around until they're called again.
Typically, extras aren't "discovered," and being an extra isn't necessarily the best way to move up the acting ladder. However, being an extra does provide valuable experience in learning about on-set rules, technical terms and overall how a movie set works. Often extras are kept separate from the principal actors, and there are often rules on the set about extras talking to the big-name extras.