Well list some actors that have had a challenge in a movie, because to me it seems like all roles are pretty easy to play.
Denzel is pretty good, but I didn't like him that much in 'Inside Man'.. He seemed fake.. :/
Norton's accent in 'The Illusionist' also seemed to be off in parts of the movie, it wasn't constant.
They are both great actors, but I bet if either one of them said something in public that you didn't like, you would immediately deem them bad actors.