Depends on the contest, could be an umpire or referee
That depends on what court you're referring to. The person may be a Judge, Magistrate, Justice of The Peace, or Referee.
An esthetician is a person that acts something like a dermatologist.
A person who acts without thinking is usually called "impulsive."
this person is called a mime.
A thespian.
Not surprisingly, a Judge and Jury act as judge and jury.
A person who acts may be called an actor or an actress. A person who acts on behalf of another is an agent.
If it is a rich person who donates money, it is called a philanthropist.
The people that perform the dangerous acts in films are called stunt men or women.
A person who acts as a reserve to a leading actor is called an understudy. They are typically prepared to step in and play the role if the leading actor is unable to perform.
the judicial branch
If it is a male, it is called an actor, and (if female) an actress.