Aside from actors such as Bill Roach aka Ken Barlow, incidentally, his son Linus is a good actor, there are actors, I use the word loosely who only ever play themselves. I'm thinking of royal descendant, Danny Dyer, he's always the same, could he ever be anyone else? I did think at one time Hugh Grant could only ever play floppy haired posh boys, but he did rather a splendid turn as Jeremy Thorpe.
I'm always impressed by actors who can turn their hand to a whole breadth of characters, always thought Daniel Day Lewis typified amazing versatility.