The first step to figuring out a story is your characters. And with each book, I go through hundreds of sketches that aren’t used. Here are a few that I did in the early stages of That Is Not a Good Idea! for Hungry Fox, Plump Goose, and the Baby Geese.
Purple Fox: After I have a sense of a story and their characters, I start to make test sketches. It's my hope that the technique (the brushes, pencils, markers, or crayons) will give me an insight into who the characters really are.
Baby Geese: This process goes on for a while (sometimes weeks, sometimes months). Even the character stylings that don't work help. The covering-the-eye pose from this test was used in the final book, even though the characters look completely different.
Goodwolf Sketch: We use butcher-paper as our tablecloth, so that our family can doodle every night after dinner. (You can see some examples on the Mo Willems Doodles blog.) All of the characters seem to have taken over this night's dinner (I saved the drawing for later reference).
Test Background: Once the characters started to come into focus, it was time to go through the same process with the backgrounds...