Answer: Peter Clemenza
Richard Castellano, who played Clemenza in 'The Godfather,' was set to reprise his role for the sequel, but he apparently demanded several perks (an inflated salary, the right to write his own dialogue, etc.) that Francis Ford Coppola found unacceptable...so, at the last minute, Clemenza was written out of the movie (via the convenient lethal heart attack) and a brand-new character, Frank Pentangeli, was created to fill Clemenza's shoes as the 'star witness' for the Senate hearings on Mafia activity.