They know what lines have to be said and have to be said this way. If your character would lie, you lie. The killer would say, 'no, I didn't do it'. The actors have to be very comfortable with their characters. They have to understand them well to be able to answer questions from the audience about their character's actions.
Bond says that when a different actor plays the role, it changes the whole play.
For actors, this type of theater can be grand. Anna B. Dalton is a member of the troupe that is directing a show for Playmakers and Co. in Newport so could not be in the current production. Because we work as a group, the characters grow out of the group as opposed to having a director that imposes their vision of the character on your character. You break the wall. You make them a part.