understudy vs actor

understudy and actor both are nouns.

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

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

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

understudy (noun)	actor (noun)
an actor able to replace a regular performer when required	a theatrical performer
	a person who acts and gets things done

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