understudy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• intransitive verb To study or know (a role) so as to be able to replace the regular performer when required.
• intransitive verb To act as an understudy to.
• intransitive verb To be engaged in studying a role so as to be able to replace the regular performer when required.
• noun A performer who understudies.
• noun A person trained to do the work of another.

from The Century Dictionary.

• To memorize (a part) as an understudy.
• noun Theat., one who has made a special study of a particular part, and is capable of playing that part at a moment's notice in the absence of the actor or actress to whom it is usually assigned.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who studies another's part with a view to assuming it in an emergency.
• verb (Theater) To study, as another actor's part, in order to be his substitute in an emergency; to study another actor's part.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb to study or know a role to such an extent as to be able to replace the normal performer when required
• verb to act as an understudy (to someone)
• noun a performer who understudies; a standby

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an actor able to replace a regular performer when required
• verb be an understudy or alternate for a role