understudy vs thespian

understudy and thespian both are nouns.

Word	Noun	Adjective	Verb	Adverb
understudy	Yes	No	Yes	No
thespian	Yes	Yes	No	No

As nouns, thespian is a hypernym of understudy; that is, thespian is a word with a broader meaning than understudy:

Other hypernyms of understudy include actor, histrion, player, role player.

understudy (noun)	thespian (noun)
an actor able to replace a regular performer when required	a theatrical performer

understudy (adjective)	thespian (adjective)
	of or relating to drama

understudy (verb)	thespian (verb)
be an understudy or alternate for a role