I believe this deserves another thread. These are the letters from Francis Ford Coppola to Richard Castellano after Castellano had set some conditions.
Remark: Richards wife, Ardell, had a deal to work with Coppola. Obviously, she wanted more rights. Additionally, she was furious about how Coppola forced Castellano to gain weight up to 325 pounds, which seriously decreased his health conditions. But the way Ardell describes it in her book and the way Coppola answers, you can tell that Ardell and Richard were unable to communicate.
And: Money wasn't the real reason. Coppola would have made Castellano a millionaire. And his position behind Pacino and Duvall was justified.







Her you find Ardell's book. It's really worth to read, also because she shows how the script changed.