stand-in

(noun) someone who takes the place of another (as when things get dangerous or difficult). Synonyms: substitute, relief, reliever, backup, backup man, fill-in. Examples: "The star had a stand-in for dangerous scenes." "We need extra employees for summer fill-ins."

(verb) be a substitute. Synonyms: substitute, sub, stand in, fill in. Examples: "The young teacher had to substitute for the sick colleague." "The skim milk substitutes for cream--we are on a strict diet."