Angela appears kind of unstable and paranoid when she first jumps to the conclusion that Kat is possessed when Kat only seemed depressed and withdrawn (understandably) after the death of her close friend. But after Angela reveals that she is in fact Regan MacNeil (the possessed girl from the film) her paranoia and prescience make complete sense, even though she was wrong about which daughter was compromised.
1971 novel:
In the book, when Lt. Kinderman speaks with Karras for the last time (letting him know, circumspectly, that he knows what's going on) he asks him to tell Karl that his daughter Elvira is in a clinic and is all right. Karras looks for Karl, but doesn't find him before the last confrontation. Presumably Elvira will eventually contact Karl herself. At this point, will Karl reveal to his wife that Elvira has been alive all this time?
Pazuzu's rage when Father Merrin dies. Father Merrin was, presumably, a good, faithful priest. His death means he is likely in Heaven (or will be relatively soon if you believe in Purgatory,) the one place Pazuzu will never be able to reach him.
The film
The film lost something by having Pazuzu giggle when Merrin died rather than yelling at him to come back. On the other hand, it could be interpreted as excitement that the much more vulnerable Father Karras is now on his own against it, since Pazuzu's revenge angle against Merrin is downplayed in the film in favor of Karras's internal conflict.
The spider-walk scene in the book is less terrifying at first than completely, inhumanly alien. Blatty's description doesn't sink in for a minute. Then it hits you.
Karras blames himself for not having the money to take care of his mother because he's a priest and not a psychiatrist. But the church sent him to college. He never really had the option of the former without the later. He blames himself for something that he couldn't have averted. And so does his uncle.
