Like mustachepete said, it's rare when a great novel translates to the big screen and makes a great film. Quite often, so much novel material has to be left on the cutting room floor that much of the original meanings behind the novel are left behind as well. The Dune movie comes to mind when I think of it. The movie was fairly well made, but so much material had to be left behind that many of the themes were lost.

Godfather managed to avoid most of that.

Puzo could've left the Lucy/Jules stuff right out as far as I'm concerned. I skip those chapters when I read the novel.

Perhaps that's why Puzo put that in along with the Johnny Fontane saga and the backstories. He hoped that one day a filmmaker would want to make a film based on his novel and he knew they'd need some material to cut out.