This discussion underscores a problem in GF and II: Fredo is made out to be so pathologically weak and stupid as to be a near-charicature. He was, to be sure, a minor character in GF and in the novel, and FFC and Puzo were wise to give him a major (even pivotal) role in II. But I think his character was distorted from the get-go. In the novel, we see him described as a dutiful son. He has two moments of weakness: his nervous breakdown when Vito was shot, and the gaffe with Moe Green. But in GF, the first thing we see of Fredo is that he's dead drunk at Connie's wedding. Then, he's not just taking Moe's side, he's also setting Michael up for debauchery--and giving Moe a back rub. The flashback scene ("The Japs didn't know it was Pop's birthday") is just gratuitously stupid.

You might argue that Fredo's treason in II (especially the boathouse outburst) was the more shocking because of how weak and stupid he appeared earlier. But I think he could have been a more effective character if he'd been given a little more balance--especially in the person of such a fine actor as John Cazale.