standby vs histrion

standby and histrion both are nouns.

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

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

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

standby (noun)	histrion (noun)
something that can be relied on when needed	a theatrical performer
an actor able to replace a regular performer when required

standby (adjective)	histrion (adjective)
ready for emergency use