In my introduction (see below), I speculated that Frank Herbert wrote the end of the book first (about the last 10%). Then he wrote the beginning. Then, after a lot of research and invention, he wrote the rest. I am basing my guesses on two indicators:
(A) Writing style. There are some quirks in the beginning and the end that you will rarely find in the middle. My favorite of these is the way characters move. They are forever “crossing” to each other, as in: Paul crossed to Jessica.
“Crossing” is a common stage-play direction, but I think it appears rarely in novels because it is undescriptive. Did Paul walk fast or slow? Did he slouch? Was he eager? Actors and directors can figure that sort of thing out, but readers want to know. In the middle of the book, characters “cross” less often.
(B) Character: If you read Herbert’s other writing, you will marvel at his rich invention of Sci-Fi technology and how he uses it to create stories. But you will notice how rarely his characters come to life. We are lucky that Herbert did better when he wrote Dune. However, it is quite noticeable that the characters in the final scenes have hardly any presence at all. Paul Muad’Dib might be forgiven for becoming a force of nature instead of a person, but it is still striking that at the end, he is all bombast. Baron Harkonnen has no character in his last scene. Feyd Rautha is even more of a stereotype. Thufir Hawat and Jessica are cardboard.
I think that Herbert developed his characters over time, after he wrote the end of the novel.