Fantastic question; first, portraying imaginary characters is no "convenience" for actors. In fact, one way to explain it to a small child would be to say that great actors are the ones who make it look easy. But it is an extremely difficult process, and much of the answers you are looking for should be discovered on your own.
Acting, by Sandy Meisner's definition, is living truthfully under imaginary circumstances. "Great" acting is revealing the full life of the character through those circumstances. And the actor must understand his own unconscious because, to oversimplify it, because it is part of who he is. Consider the Greek maxim, "know thyself." By having a thorough knowledge of self, obtained through many years of training and experience, is how the actors you mentioned got to be great.
· 6 years ago