I think that the Fredo character may have put Coppola in a serious bind. The Freddy of the novel is, as TB says, a minor character, but that's of necessity because he is designed not to have an inner life. Lack of passion and introspection define him, so there's nothing to emerge on film.

So they replace that guy with the weak and stupid persona. Cazale knocks that out of the park, and Fredo gets upped for a big role in the GF2 script. But, that stifles the script in a way that leaves some of our neverending questions not just unanswered, but unanswerable. If Fredo says his boys killed the shooters, if he says that the plan was for Mike to be kidnapped, then when Fredo is shot in the back of the neck the audience will applaud. Coppola can't have that and still have his tragedy, so no one can tell the audience what actually happened.