"Extras" is about an out-of-work actor (Gervais) who struggles for work when he keeps losing his parts to stars like DeNiro. However, he insists that they're "just lucky". Andy Millman has given up his day job to be an actor, but finds he just can't land the big parts. In fact, he's never even had a speaking role, so spends much of his time hanging around on set with fellow extra and best friend, Maggie Jacobs. Andy envies the A-list stars, but never misses an opportunity to ingratiate himself with them, hoping it will lead to that first elusive line of dialogue.
Contrary to vicious rumors, it is not done by bussing in a couple of people and aks them to stand in a parking lot to be chosen one by one by an over worked first AD. Casting extras is actually done via sophisticated computer databases and telecommunications between casting directors and production companies.
It is a relatively new way of casting extras, only developed within the past couple of decades. Your image and profile are kept on file along with many thousands of others who would be actors, and production companies call in requesting specific numbers of specific types. This information is posted daily on a voicemail system, which you then access, listening for calls requesting your type. You are the last link in this method of casting extras, as it is your call to the number on the recording that makes the final connection.