THESAURUS
act for
advance, answer for, appear for, assist, back up, be instrumental, change places with, commission, crowd out, cut out, deputize, displace, double for, facilitate, fill in for, forward, front for, ghost, ghostwrite, go between, mediate, minister to, pinch-hit, pinch-hit for, promote, relieve, replace, represent, serve, speak for, spell, spell off, stand in for, subrogate, subserve, substitute for, succeed, supersede, supplant, swap places with, understudy, understudy for
