S: (n) stand-in, substitute, relief, reliever, backup, backup man, fill-in (someone who takes the place of another (as when things get dangerous or difficult)) "the star had a stand-in for dangerous scenes"; "we need extra employees for summer fill-ins"
Verb
S: (v) substitute, replace, interchange, exchange (put in the place of another; switch seemingly equivalent items) "the con artist replaced the original with a fake Rembrandt"; "substitute regular milk for fat-free milk"; "synonyms can be interchanged without a changing the context's meaning"
S: (v) substitute, sub, stand in, fill in (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"
S: (v) substitute, deputize, deputise, step in (act as a substitute) "She stood in for the soprano who suffered from a cold"
Adjective
S: (adj) utility, substitute (capable of substituting in any of several positions on a team) "a utility infielder"