understudy

In theater, an understudy, referred in opera as cover or covering, is a performer who learns the lines and blocking/choreography of a regular actor or actress in a play. Should the regular actor or actress be unable to appear on stage because of illness, emergencies or death, the understudy takes over the part. Usually when the understudy takes over, the theater manager will make a relevant announcement prior to the start of the performance. First coined in 1874, the term "understudy" has more recently generally been applied only to performers who can backup a role, but still regularly perform in another role, usually a minor one.