fill in vs fill out

fill in and fill out both are verbs.

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

As verbs, fill in and fill out are synonyms defined as:

Other synonyms of fill in include complete, make out.

fill in (verb)	fill out (verb)
supply with information on a specific topic	become round, plump, or shapely
write all the required information onto a form	make bigger or better or more complete
represent the effect of shade or shadow on	write all the required information onto a form
be a substitute	make fat or plump
	line or stuff with soft material
	supplement what is thought to be deficient