Cartoons are like Christmas trees. People put a lot of energy into decorating their trees because everyone enjoys the fancy part of the decoration—the baubles and sparkles. But to decorate a tree requires that it have branches and needles, and both of these require the tree to have a trunk and, ultimately, roots, even if we never see them. There are lots of examples in the world of animation where the finished product is a scrawny, impoverished creation because it never had a trunk or roots. And some works—with trunks supported by bamboo, or bits of iron and plastic soldered together—that may have been intended to be new and novel, end up being just weird, jerry-built exhibitions that are hardly enjoyable to behold.

Hidden among the many illustrations drawn at the conceptual stage, among all the materials originally used in discussing the chaotic and disparate ideas, lies the real story we want to create. What's important with scenarios is to steadily whittle away at them, to reveal a story with a solid trunk or core.

Source: Hayao Miyazaki's Memoir "Starting Point 1979-1996"