After a while I think these actors start to forget the difference between pretending to be a character and actually being that character. Maybe this is why their self images are out of sync with reality and inclining toward delusion.
Having been an actor, I can tell you that actors have a notoriously troubling occupational hazard, a perpetually endangered “sense of self”, in the area of identity as it’s known and felt by non-actors-—their very success is determined by how well they can convey BEING SOMEONE ELSE, how far they’re willing to go into other identities, over and over and over, (assuming they get the golden opportunities to do it over and over and over).Knowing what kind of round-the-clock schedule is sometimes required for both movies and television, it’s a wonder they have much time to develop their own personalities.
Peter Sellers was a prime example of this: a genius performer, but failure as father and husband, out of touch with and bored with “himself”, that self having become like a character he didn’t get a chance to play.