fill out vs write down

fill out and write down both are verbs.

Word	Noun	Adjective	Verb	Adverb
fill out	No	No	Yes	No
write down	No	No	Yes	No

fill out (verb)	write down (verb)
become round, plump, or shapely	reduce the estimated value of something
make bigger or better or more complete	put down in writing; of texts, musical compositions, etc.
write all the required information onto a form
make fat or plump
line or stuff with soft material
supplement what is thought to be deficient