I've been thinking recently about how often we talk about the book informing understanding of the movie. It's occurred to me that in the audience for the original release there must have been an awful lot of people who had read the book: 9 million copies had been sold just in 1969-71, and some of those were read by more than one person. Box office in 1972 was $81 million, which would be about 40 million admissions at the average ticket price then, but I think it probably had relatively few discount admissions, and then people could have gone multiple times. So it wouldn't surprise me if a third of the people in any theater had already read the book and were filling in any gaps on their own.

It's also occurred to me that I don't have any idea if I've ever seen the first two movies on a big screen. I was 13 when The Godfather was released, and mostly into sports and comedy. I remember seeing the NBC miniseries version in the late 70s. I was in college in the late 70s and early 80s, we had a great repertory cinema just off campus and tons of on-campus movies. I went to a lot of movies, seldom in first run, so it seems almost certain I saw them, but just don't recall being in the theater. So maybe I'll do that some day.