Brad Pitt always just plays Brad Pitt, when he is cast it is usually because the producer / director whoever wants exactly that; A-lister, star quality. His name will attract an audience, but a good actor by skilful acting methodologies he is not. Certainly not when taken comparatively against an experienced character actor say like Michael Sheen, who tends to disappear into the role.
Tom Cruise is a fair example of someone who was able to move away from that sort of "just a big name on a billboard" status to someone who can actually take on and accomplish a role; see: War of the Worlds, Collateral, or Tropic Thunder.