1. [n]  an actor able to replace a regular performer when required
2. [v]  be an understudy or alternate for a role

Understudy
When Amy and her best friend Robyn are involved in a car crash in which Robyn dies, Amy assumes her identity and becomes the rich girl she has always wanted to be. Trouble is, she`s in love with Robyn`s brother--who is now, of course, her brother. And the man she finally does marry is drop-dead handsome but, unfortunately, gay....

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1. \Un"der*stud`y\, v. t. & i. (Theater)
   To study, as another actor's part, in order to be his
   substitute in an emergency; to study another actor's part.
   

2. \Un"der*stud`y\, n.
   One who studies another's part with a view to assuming it in
   an emergency.