As far as the GF screenplay goes, it may have been Puzo, it may have been FFC, or it may have been a meeting of minds between the two of them. But, you're correct - it definitely was not in the novel.

As a matter of fact the line "Luca Brasi sleeps with the fishes" is not in the novel in that exact form. They improved it for the film, IMO.

What I particularly noticed in rereading the book was that several lines, while they were said in the book, were said by different characters in the movie. Not always the best decision, I feel. (Particularly the line about no Sicilian being able to refuse a request on his daughter's wedding day. Michael in the book, Tom in the film.)

Signor V.

"For me, there's only my wife..."

"Sure I cook with wine - sometimes I even add it to the food!"

"When life itself seems lunatic, who knows where madness lies?"

"It was a grass harp... And we listened."

"Do any human beings ever realize life while they live it? Every, every minute?"

"No. Saints and poets, maybe... they do some."