Understudy v. t. & i. : To study, as another actor's part, in order to be his substitute in an emergency; to study another actor's part.

Understudy n. : One who studies another's part with a view to assuming it in an emergency.

Example:
What are the chances an Understudy will be able to perform?

Taxonomy
understudy, standby, actor, free encyclopedia, theater, performers, understudies, leading actor, actress, lead role, The Swing Of Things, lead actor, on stage, theater manager, broadwayworld, search, Unsourced, choreography.

Defination from the web:
understudy takes over the part. ... Several actors made their name in show business by being the understudy of a leading actor and taking the role over for several performances: ...