When an actor appears in the background of a scene, this is what’s known as an Extra or “Background Performer”. This is how SAG (The Screen Actors Guild) defines it.

The role of an extra is just like it sounds. It gives the scene a realistic look – whether its a restaurant, or a football game, or a street scene – filling up the scene with realistic looking people gives an authenticity to the scene.

Many times during the filming, they will use the actual sound of the scene. This is particularly true if they are filming a city scene or something on location. But many times, a scene can be filmed on a soundstage, or even on a set on a studio lot, and they can add in sound and effects later. But it’s the people that give a scene that authentic “crowd” look.

 Read the whole article on being an extra on the web site