There's no way to determine if it's real or not. He's a psycho, not insane, so I never had any reason to believe he imagined the stuff, except at the end with his lawyer. But I put that down to the lawyer really not knowing that he was talking to his client--like with everyone else in the movie not even knowing the names of the colleagues they were dining/drinking with, as an exaggeration of that group of society's selfcentredness.
