This is one of the most oft-asked questions on these boards. Many (most?) here believe Fredo when he said, "I swear to God, Mike, I didn't know it was gonna be a hit." He's too weak, too stupid, too helpless, too easily misled by Roth, to be in on the attempted murder of his own brother. But then you have to take into account that, just before the shooting, Kay notices that the drapes in her and Michael's bedroom are open. Who opened the drapes, if not Fredo? And if he did, to what purpose did he open the drapes? So the gunmen could sneak a peek at Kay undressing for bed?