fill-in vs stand-in

fill-in and stand-in both are nouns.

Word	Noun	Adjective	Verb	Adverb
fill-in	Yes	No	No	No
stand-in	Yes	No	No	No

As nouns, fill-in and stand-in are synonyms defined as:

fill-in and stand-in: someone who takes the place of another (as when things get dangerous or difficult)

Other synonyms of fill-in include backup, backup man, relief, reliever, substitute.

fill-in (noun)	stand-in (noun)
someone who takes the place of another (as when things get dangerous or difficult)	someone who takes the place of another (as when things get dangerous or difficult)