I always had noticed that the movies seemed to play Fredo up as the "dumb brother" more then the book.

You never seem to see Puzo write that Fredo was specifically weak or stupid. He never uses those exact words, something more vague like "obediant" implying that he can't think for himself, or "not ruthless enough" implying that he didn't have the courage(?) of Michael or Sonny.

It seems almost like Puzo attempted to avoid simply making Fredo a two dimensional character, the stupid brother, and tried to flesh him out more and make him complex. Where as the movies just go straight for the weak/stupid angle.