Your main responsibility is to portray a person, making sure to really get their motivations and essence across to an audience. To do this you use different techniques like, say, a British accent or specific movements which give nonverbal cues about your character. The best actors make you truly believe they are who they’re playing, even if you’ve seen them perform in other roles before. An actor’s job is to fully represent the character they are playing and to convince their audience that character is real by engaging them and drawing them into the story. This applies to major roles and smaller parts, as all roles combine to create a successful production.
