With Phantom Menace, he could cut together scenes from the film to give us a general idea of what he was thinking; he could also put big red slashes over Jar Jar's face. So much of the first film had been changed (and the second one was so terrible) there was almost nothing for him to use. That means storyboards. The result is not only breathtaking, it is a lesson in story editing.
What If Episode I Had Been Good?
What If Episode II Had Been Good?