6. Fredo's continued silence - After Michael "forgives" Fredo at Mama's funeral, Fredo stays on the compund but he keeps to himself. He never discloses that he knows Pentangeli is alive, or that the Senate lawyer is on Roth's payroll. Even when everything blows up at the committee hearing he lies to Tom Hagen and says he knows nothing, yet when Michael confronts him, he tells Michael he knows Pentangeli is alive and "they've got him" and that the Senate Lawyer belongs to Roth. I have argued this before, I think Fredo did know it was going to be a hit, and he did know Roth's motives. I think we have a consensus that Fredo opened the drapes, so he had to know. Still he continued to lie to Michael about it.
Why? I think Roth promised Fredo he would be head of the Corleone family after Michael's assassination. That was the "something in it for me." It would rectify Fredo's being passed over. And btw it is the reason Michael was right to kill him.