fill out vs fill in

fill out and fill in both are verbs.

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

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

Other synonyms of fill out include complete, make out.

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