Overview
A background actor or extra is a performer in a film, television show, stage, musical, opera, or ballet production who appears in a nonspeaking or nonsinging (silent) capacity, usually in the background (for example, in an audience or busy street scene). War films and epic films often employ background actors in large numbers: some films have featured hundreds or even thousands of paid background actors as cast members (hence the term "cast of thousands"). Likewise, grand opera can involve many background actors appearing in spectacular productions.
On a film or TV set, background actors are usually referred to as "supporting artists", "junior artists", "atmosphere", "background talent", "background performers", "background artists", "background cast members", "talent", "background friends", or simply "background", while the term "extra" is rarely used and is often considered derogatory. In a stage production, background actors are commonly referred to as "supernumeraries". A more archaic theatre term is "sword carriers". In opera and ballet, they are called either "extras" or "supers".
Casting
Casting criteria for background actors depend on the production. It is not entirely true that background cast members require little or no acting experience, as any type of unrealistic portrayal must include some form of imagination and acting. Punctuality, reliability and the ability to take direction also figure prominently for these cast members. Background actors are generally selected on short notice, after all other preparations for the shoot have been finalized.
From Wikipedia (CC BY-SA 4.0).