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.
extra
power
Steve McQueen
Audition
Jake Gyllenhaal
Substitute for an actor.
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.
I used applesauce as a substitute for oil in the recipe to make it healthier.
In the game of Scrabble, you can use the word "exert" as a substitute for "apply effort".
Thespian is another word for an actor or actress.
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Good that can be used in place of another good.
substitute
replacement
what word can I substitute for constitutes
Generally an actor with no dialogue is called an extra.