I heard or read somewhere that Mario Puzo said that Pacino's depiction of traumatic grief, the "Silent Scream", was the single most effective piece of acting that he had ever seen. It is made more affecting by having the Vincent Mangano character, Michael's would-be successor, witness this scene helplessly, grimly looking on at the personal devastation that has come to someone who has wielded criminal power for so long. This is the moral center of the entire Godfather film trilogy. The payoff, if you will. And it rightly hurts.