Another question related to this topic.

The novel says:
He [Sonny] said eagerly, “The Tattaglia family guarantees the return of our investment without any percentage from us?”

In the film he acts as if he feels insulted by Sollozzo: "Aw, you're telling me that the Tattaglias guarantee our investment?"

My point: A greedy Sonny is obviously "a ch*nk in the Don’s fortress", while the insulted Sonny isn't necessarily.

Was this little twist in interpretation of the script intended by FCC and/or James Caan?