In the novel, Kay came to believe, at least temporarily, that Felix was indeed Sollozzo and Mac's killer.

I always found it interesting, that in the novel, it was Kay who came to the mall at the suggestion of Mama Corleone. Then, after "surprising" Michael, she went to his apartment in the city with him, where they had their "in five years the Corleone family will be legitimate" conversation.

I wonder what made FFC change the storyline to Michael going up to New Hampshire to find Kay?