fill vs fill in

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

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

fill (noun)	fill in (noun)
a quantity sufficient to satisfy
any material that fills a space or container

fill (verb)	fill in (verb)
become full	supply with information on a specific topic
make full, also in a metaphorical sense	write all the required information onto a form
plug with a substance	represent the effect of shade or shadow on
fill to satisfaction	be a substitute
eat until one is sated
fill, satisfy or meet a want or need or condition or restriction
assume, as of positions or roles
appoint someone to (a position or a job)
occupy the whole of