understudy vs role player

understudy and role player both are nouns.

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

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

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

understudy (noun)	role player (noun)
an actor able to replace a regular performer when required	a theatrical performer
	a person who makes deceitful pretenses

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