Some location scenes for a comedy film will be shot at my children’s school over the holidays and a casting company is advertising for extras at $75/day. Naturally, my kids want to get picked. What do casting people look for in movie extras? Tall, short, medium? Fat, thin, medium? Do they want people who blend in or people who stand out? Are there any Dopers in La la land who can provide the straight dope?
I’m afraid it depends entirely on the casting director and nature of the production.
So to answer your question: It can be any or all of the above.
Generally, casting calls will have general info about the production and the basics of what they are looking for:
E.g. “A family drama set in a racially segregated school system. Looking for extras between the ages of 8-12 of African decent.”
Then it would be up to the casting department to weed out more specifically the extras they want and you’re at the mercy of their whim. (The casting department in turn gets info from the director – so if the director says “I want skinny kids that look poor” then the casting dept. will weed out the extras accordingly.)
Otherwise, the production company calls an agency and will say “we need a bunch of skinny, bald guys” and the agency will contact the likeliest candidates in their database that are skinny and willing to shave their heads.