In the book Fredo goes into shock after seeng his Father shot and Michael thinks about how Fredo was always the most obedient son, but the Don had given up on him being important in the business because he was 'not quite smart enough' and 'not quite ruthless enough. He was too retiring a person, did not have enough force.' frown
It doesnt really go into this in the film.
I think Fredo is one of my favourite characters.

"My wife was weeping before she fell asleep, outside my window I saw my Caporegimes coming to the house and it is midnight. So, Consigliori of mine, I think you should tell your Don what everyone knows."