Are you talking about an understudy? Understudies are people who come to rehearsals and then if the actor that was formerly hired cannot perform (due to sickness, quitting, ect.) then the understudy takes their part. I think that they are called understudies because they study the actor's lines (hence study) and are underneath the position of the actor. I hope I helped.
Substitute for an actor.
you can not substitute me. The director Substituted lead actor.
The substitute teacher is horrible.I need to substitute something with the nuts in this recipe.The manager called the substitute onto the pitch.
Yes. The substitute runner is called the "pinch runner".
An official who takes the position of another official is usually called a substitute, reserve, or replacement official.
Thespian is another word for an actor or actress.
Good that can be used in place of another good.
Generally an actor with no dialogue is called an extra.
One possible substitute for "their" is "his" or "her" depending on the gender of the person/people being referred to. Another substitute can be "its" when referring to a non-human object or entity.