fill in vs fill-in

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

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