fill vs bolster

fill and bolster both are nouns.

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

As verbs, bolster is a hyponym of fill; that is, bolster is a word with a more specific, narrower meaning than fill:

fill (noun)	bolster (noun)
a quantity sufficient to satisfy	a pillow that is often put across a bed underneath the regular pillows
any material that fills a space or container

fill (verb)	bolster (verb)
become full	add padding to
make full, also in a metaphorical sense	support and strengthen
plug with a substance	prop up with a pillow or bolster
fill to satisfaction
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