I didn't read The Haunting of Hill House until sometime early in the 1990's.
I lived in Georgetown in the late '70s about four houses down from the steps.
Horror does not interest me, and so I know little of its practicioners, old or current.
Well, the research into it affected me. And the novel, it very much strengthened my faith.
I'd sold the book first. Actually to a paperback publisher. I had nothing. I just had the idea.
I'm not aware that I was consciously influenced by any director, though these things often happen unnoticed, submerged in the unconscious.
And the sad truth is that nobody wants me to write comedy. The Exorcist not only ended that career, it expunged all memory of its existence.
I get cassettes near Academy Award time of every movie that's made that thinks it has some kind of chance for a nomination - that's when I watch my movies.
I tried to make every bit of it as creepy as I could. And I had the same response you do. I feel the same way. The hospital scenes, that procedure was so real.
I have never read horror, nor do I consider The Exorcist to be such, but rather as a suspenseful supernatural detective story, or paranormal police procedural.