bit part vs role

bit part and role both are nouns.

Word	Noun	Adjective	Verb	Adverb
bit part	Yes	No	No	No
role	Yes	No	No	No

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

Other hypernyms of bit part include character, part, persona, theatrical role.

bit part (noun)	role (noun)
a small role	the actions and activities assigned to or required or expected of a person or group
	normal or customary activity of a person in a particular social setting
	what something is used for
	an actor's portrayal of someone in a play