Here's a fantastic video essay exploring how Star Wars' editors recut and rearranged Star Wars: A New Hope to create the cinematic classic it became.
In film or video, an over the shoulder shot - AKA an over shoulder, ab tu, OTS, or even third-person shot - gives the perspective of looking over the shoulder of another person. The camera is positioned behind the back of the shoulder and head, however, their head and shoulder are used to frame the image.
The OTS shot is used often when two characters are having a discussion, usually following a wide establishing shout which helps the audience members get a sense of ‘where’ the exchange is taking place. The where in this example is the setting.
Think of all the people you see in the background at the diner in Seinfeld, Cheers, or even the Stormtroopers in Star Wars.
They say nothing, or pantomime gibberish to each other to make it look like they’re having a conversation.
Famous Extra Fact: James Michael Tyler was a ‘background’ actor in NBC‘s Friend’s café, Central Perk, when one day in the second season he was given a line, and suddenly he was the character Gunther on the show... for ten seasons!
The Assistant Director usually corrals and directs the background actors.
If you're interested in becoming an extra, there are many websites (try this one!) and resources out there. Remember that this is a profession, and the days can often be long, and in the elements. It's not for tourists. And also note that asking for autographs is extremely unprofessional and might get you tossed off the set.