actor — An actor is someone whose job is acting in plays or films. 'Actor' in the singular usually refers to a man, but some women who act prefer to be called 'actors' rather than 'actresses'.
agent — A chemical that has a particular effect or is used for a particular purpose can be referred to as a particular kind of agent.
confederate — Someone's confederates are the people they are working with in a secret activity.
contractor — A contractor is a person or company that does work for other people or organizations.
defendant — A defendant is a person who has been accused of breaking the law and is being tried in court.
participator — to take or have a part or share, as with others; partake; share (usually followed by in): to participate in profits; to participate in a play.