When I illustrate a cover or a book, I draw upon what the author tells me; that's how I see my responsibility as an illustrator. J.K. Rowling is very descriptive in her writing — she gives an illustrator a lot to work with. Each story is packed full of rich visual descriptions of the atmosphere, the mood, the setting, and all the different creatures and people. She makes it easy for me. The images just develop as I sketch and retrace until it feels right and matches her vision.