Fredo was a minor character in the novel, hence a minor character in GF. GFII was not envisioned when GF wrapped. To flesh out the plot--and no doubt to take advantage of John Cazale's considerable acting skills (he'd made "The Conversation" with FFC between GF and II)--FFC and Puzo wrote a significantly larger role for him in II. As I said earlier, I believe they painted Fredo too broadly as a dunce. But that dunce quality helped make his betrayal of Michael all the more dramatic, and his outburst in the boathouse into one of the best scenes in the Trilogy.