understudy

n. actor trained to act in another actor's role as a substitute, person trained to substitute for another

v. study a role in order to replace the regular actor in that role when necessary (theater), act as an understudy

An understudy is a theatrical term for someone who learns the lines and blocking/choreography of a leading actor or actress in a theatrical play. Should the lead actor or actress be unable to appear on stage because of illness or accident, the understudy takes over the part. Usually, when the understudy takes over, the theater manager will make the announcement prior to the start of the performance. Several actors made their name in show-business by being the understudy of a leading actor and taking the role over for several performances (for example, Anthony Hopkins with Sir Laurence Olivier).
See more at Wikipedia.org...

Noun
1. an actor able to replace a regular performer when required
(synonym) standby
(hypernym) actor, histrion, player, thespian, role player
(derivation) alternate
Verb
1. be an understudy or alternate for a role
(synonym) alternate
(hypernym) memorize, memorise, con, learn
(derivation) standby

(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.
  

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

Synonyms and related words:
act for, advocate, agent, alter ego, alternate, alternative, amicus curiae, analogy, answer for, appear for, attorney, back up, backup, backup man, bit player, champion, change, change places with, changeling, commission, comparison, copy, counterfeit, crowd out, cut out, deputize, deputy, displace, double, double for, dummy, equal, equivalent, ersatz, exchange, executive officer, exponent, extra, fake, figurant, figurante, figurehead, fill in for, fill-in, front for, ghost, ghostwrite, ghostwriter, imitation, lieutenant, locum, locum tenens, makeshift, metaphor, metonymy, mute, next best thing, paranymph, personnel, phony, pinch hitter, pinch-hit, pinch-hit for, pleader, procurator, proxy, relief, relieve, replace, replacement, represent, representative, reserve, reserves, ringer, second, second in command, second string, secondary, sign, spares, speak for, spear-carrier, spell, spell off, stand in for, stand-in, standby, sub, subrogate, substituent, substitute, substitute for, substitution, succedaneum, succeed, supe, super, supernumerary, supersede, superseder, supplant, supplanter, support, supporting actor, supporting cast, surrogate, swap places with, symbol, synecdoche, third string, token, understudy for, utility man, utility player, vicar, vicar general, vice, vice-president, vice-regent, vicegerent, walk-on, walking gentleman