What do you call a character that acts as a prop in a film? In my understanding, prop is used for objects, but is there a word for human characters acting as props?
A person used as a prop in the movie world is an extra. Lexico has
1.2 A person engaged temporarily to fill out a crowd scene in a film or play.
the film used an army of extras
The Wikipedia definition is
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