backup man vs fill-in

backup man and fill-in both are nouns.

Word	Noun	Adjective	Verb	Adverb
backup man	Yes	No	No	No
fill-in	Yes	No	No	No

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

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

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

backup man (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)