fill-in vs fill in

fill-in is a noun but fill in is not a noun.

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

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

fill-in (verb)	fill in (verb)
	supply with information on a specific topic
	write all the required information onto a form
	represent the effect of shade or shadow on
	be a substitute