How to sign: 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";
Similiar / Same: stand-in, reliever, backup man, fill-in, backup, relief
Categories: compeer, equal, match, peer
Within this category: double, locum tenens, surrogate
How to sign: put in the place of another; switch seemingly equivalent items
"the con artist replaced the original with a fake Rembrandt"; "substitute regular milk with fat-free milk";
Similiar / Same: replace
Categories: change, exchange, interchange
Within this category: reduce, retool, shift, subrogate, truncate
How to sign: an athlete who plays only when another member of the team drops out
Similiar / Same: reserve
Within this category: bench warmer, pinch hitter