standby vs role player

standby and role player both are nouns.

Word	Noun	Adjective	Verb	Adverb
standby	Yes	Yes	No	No
role player	Yes	No	No	No

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

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

standby (noun)	role player (noun)
something that can be relied on when needed	a theatrical performer
an actor able to replace a regular performer when required	a person who makes deceitful pretenses

standby (adjective)	role player (adjective)
ready for emergency use