Blatty is the best writer. 🙂 There’s a part in the book when Karras is studying Regan’s EEG and he finds no fluctuations in the graph. Hysterics always seem to cause some odd fluctuations, so Karras thought it was something he could use with the Bishop as an argument that Regan was not a hysteric and therefore was conceivably possessed.
I wish, so badly, that Blatty could extend the novel. In the ‘Fear of God’ commentary, he said that he was going to write another 250 pages, up to the point when Merrin dies. So the book would have about 500 pages!