stand-in vs fill-in

stand-in and fill-in both are nouns.

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

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

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

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

stand-in (noun)	fill-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)