Frank, many people here have speculated on what Clemenza's role would have been if Castellano had signed on for II. The most-often held assumption is that he would have played out as did Pentangeli: betrying Michael. That led to a lot of hand-wringing here--how could someone we know, love and trust like Clemenza betray Michael? All I know is that, if Castellano had signed on, the script might have been written entirely differently. Perhaps Clemenza would have been loyal to Michael and helped him fight Roth, or have gotten killed in the process?

While it's too bad Paramount and Castellano couldn't come to terms, as Olivant noted, it was credible to write around him, given age, physical condition, etc. (given his condition, I wondered how he survived climbing all those stairs near the end of GF). What matters more is that Michael V. Gazzo, as Pentangeli, makes GFII.