fill-in vs backup man

fill-in and backup man both are nouns.

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

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

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

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

fill-in (noun)	backup man (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)