In Part II, when Michael is in Cuba, Fredo comes to see him with the suitcase full of cash. He introduces himself to Michael's bodyguard, who doesn't shake his hand until Michael appears to tell him that Fredo's his brother. Does this make any sense? During this period of crisis, would Michael really have a bodyguard who was such a stranger to the Corleones as to not even know who Fredo was?

Let me tell ya somethin my kraut mick friend!