Sherlock Holmes lives in a London identical to the one in which Conan Doyle’s books about him were published, except that it contained no books about him by Conan Doyle. Doctor Who couldn’t watch Doctor Who. Novels, in the very act of being created in our world, vanish from their own fictional world. Something quantumic here but God knows what.
In films this applies to actors: George Clooney can’t be a real person in a film in which he acts – just imagine if they met. Being John Malkovitch plays with this problem, by setting up a world where people can crawl down a muddy tunnel and end up in John Malkovitch’s head looking out through his eyes, and able, with practice, to override Malkovitch’s own mind to control his body. Malkovitch plays Malkovitch, and at one point himself crawls down the tunnel, entering a fantasy world where everyone is him. There’s no logical way out.
And Truman, dear Truman, is in his own world, but doesn’t know this. When he knows, he becomes someone different, and leaves.
And Buzz Lightyear does exist in the film’s world, but not in the (unique, fictionalised) way he thinks he exists: when he views the shopping aisle stuffed with rows and rows of himself, and realises that he is a toy, his existential pain beats Truman’s.
We exist in the mental worlds of others, but not our own: the subject cannot be part of the object.