How well do actors really "act" in your opinion?

I've been noticing that actors often play the same kinds of roles a lot. McLovin gets the nerdy guy roles, Bruce Willis gets the cop role, Deniro plays Italians and so on. So do actors really 'act' and play characters, or are they just playing themselves on screen?

I am starting to think it is more of the second one. It is hard to tell when you do not know actors personally though