WordNet (r) 3.0 (2006):

stand in
    v 1: be a substitute; "The young teacher had to substitute for
         the sick colleague"; "The skim milk substitutes for cream--
         we are on a strict diet" [syn: substitute, sub, stand
         in, fill in]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "stand in":
   advocate, agent, alter ego, alternate, alternative, amicus curiae,
   analogy, assistant, attorney, backup, backup man, bit player,
   champion, change, changeling, comparison, copy, counterfeit,
   deputy, double, dummy, equal, equivalent, ersatz, exchange,
   executive officer, exponent, extra, fake, figurant, figurante,
   figurehead, fill-in, ghost, ghostwriter, imitation, lieutenant,
   locum, locum tenens, makeshift, metaphor, metonymy, mute,
   next best thing, paranymph, personnel, phony, pinch hitter,
   pleader, procurator, proxy, relief, replacement, representative,
   reserves, ringer, second, second in command, second string,
   secondary, sign, spares, spear-carrier, standby, sub, substituent,
   substitute, substitution, succedaneum, supe, super, supernumerary,
   superseder, supplanter, support, supporting actor, supporting cast,
   surrogate, symbol, synecdoche, third string, token, understudy,
   utility man, utility player, vicar, vicar general, vice,
   vice-president, vice-regent, vicegerent, walk-on,
   walking gentleman