why do you think LBJ was in on it?
He seemed very happy about the whole thing, could hardly keep the grin off his face when he was sworn in. Some people would say that was just a character trait and that he was just happy to become the President, but he just seemed too happy for my liking.
Also, Jacki O's demeanour was like she knew her husband had been murdered, but there was nothing she could do about it. She showed tremendous strength on that day, but I think it was partially because she'd been expecting it and it didn't shock her.
And, Johnson restored the car that JFK was killed in, and drove around in it during his Presidency. He could have had them build him a new car, but he preferred the one that JFK died in.
I believe that Johnson knew in advance, and he played along with it because it was going to make him the President.