When you think of it, FFC had Clemenza doing alot in GF I. It was Clemenza who was emphasized at the wedding, who accompanied Sonny out to confront the FBI, who was given the Bonasera job, who came to Sonny's house after Vito was shot, who showed up with Hagen and the detectives at the hospital, and who trained Mike with the gun. Did I miss anything? Some of this emphasis is based on the novel, but still it shows the Corleones' preference for Clemenza.

I also wonder why FFC changed the fishes wording from what's in the novel?

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
"Instagram is Twitter for people who can't read."