understudy vs histrion

understudy and histrion both are nouns.

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

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

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

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

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