FFC and Puzo made Fredo almost a cartoonishly dumb character. But a silver lining in our perception of his terminal simplicity is that we were all the more shocked when he turned out to be the traitor. No matter what his role, he was pivotal to an assassination plot that very nearly worked.
Well said.
"The only wealth in this world is children. More than all the money and power on Earth." --Michael Corleone
