An actor is the one who acts in a specific role in a film. They may or not mimic, do comedy, sing, dance, do stunts, action sequences but they have to act to be called as an actor. The term of acting has come from theatre where people performed live. Actors play all kinds of roles depending on the film plot and story. They are trained professionals who may or may not have learnt acting from an acting school.
They can be the main lead or supporting actor based on their roles. Main actors or leads are expected to know dancing to some extent as they most definitely will have song sequences in the film. A movie actor has to be present for press events and other promotions. They are expected to be good role models for the public and encourage good behavior as they are seen as inspiration for the audience and society.