I was surprised when i noticed that that classic line wasn't in the novel.Mario Puzo probably added it in for the movie.It seems more likely it was him instead of FFC.Or maybe not, maybe it was FFC.

The hero and the coward both feel the same thing, but the hero uses his fear, projects it onto his opponent, while the coward runs. It's the same thing, fear, but it's what you do with it that matters. Cus D'Amato