role vs bit part

role and bit part both are nouns.

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

As nouns, bit part is a hyponym of role; that is, bit part is a word with a more specific, narrower meaning than role:

Other hyponyms of role include minor role, heavy, hero, ingenue, name part, title role, heroine, baddie, villain.

role (noun)	bit part (noun)
the actions and activities assigned to or required or expected of a person or group	a small role
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