Many people believe that the secret of a good actor is to “disappear” into a role and be a “chameleon” and play various roles very successfully. And a non-actor or a bad actor is always him/herself and plays all different roles alike or plays only one type of roles. We usually don’t think of character actors as great artists because of the same reason.
But I think what distinguishes a great actor from a good actor is that there is always a hint of that “him/herself” in the role, although they disappear into the role, you still recognize them as themselves, and while they are still chameleons, they still have some common features you can look at and recognize. You are aware that you are watching them and yet you have forgotten about everything but the role – and out of this paradox a great performance is born. I don’t know why great actors work like that, but all of them I can think of are like that.
One can name Marlon Brando, Ingrid Bergman, Lillian Gish, Robert De Niro, Al Pacino, Laurence Olivier, Daniel Day-Lewis, Catherine Deneuve, Katherine Hepburn, Bryan Cranston, James Gandolfini, Meryl Streep, Naomi Watts, and finally Philip Syemour Hoffman. All of these actors seem to become one with the role in many aspects, yet you can see some common mannerism and signature moves that extinguish the actor too. I can think only of Christian Bale who’s an actor who completely disappears into a role and I consider him great.
The same of course can be said about the great writers and painters and directors too, you recognize their work, although each work is different in a way.