If it is in his head, it’s not all in his head. Presumably the more mundane parts of his life are real, and perhaps the extreme parts are “inspired by real-life events” in a very loose way.
At least some of it has to be. Again, I draw your attention to:
FEED ME A STRAY CAT
Uh, unless you’ve got a buddy who works with these machines, if you see this phrase in the display of your ATM, you’re having a psychotic episode.
Are you talking about the book or the movie?
I think it pretty clearly was in his head in the movie.
A lot of reviewers and readers of the book rather clearly thought it was real and it inspired an attempted boycott by IIRC NOW.
I haven’t read the book so I can’t say if they were correct or not, though I remember a reviewer who clearly thought it was real listing one of the books errors that none of Bateman’s murders made the papers, which when I saw the movie led me to think the reviewer might have missed the point.
This.
To me most, if not all of it, had to be real because otherwise there wouldn’t be much of a point to the story. I read it as a comment on society.
The suspicious and blatantly obvious things that he did in front of other people should have been noticed, but they weren’t --not because it was all in his head but because the people around him are either too superficial and self-absorbed to notice, or they simply don’t want to be inconvenienced.