incidentally vs apropos

incidentally is not an adjective while apropos is an adjective.

Word	Noun	Adjective	Verb	Adverb
incidentally	No	No	No	Yes
apropos	No	Yes	No	Yes

As adverbs, incidentally and apropos are synonyms defined as:

Other synonyms of incidentally include by the bye, by the way.

incidentally (adjective)	apropos (adjective)
	of an appropriate or pertinent nature

incidentally (adverb)	apropos (adverb)
introducing a different topic	at an opportune time
of a minor or subordinate nature	introducing a different topic