An illustrator in my own mind - and this is not a truth of any kind - is someone who so falls in love with writing that he wishes he had written it, and the closest he can get to is illustrating it. And the next thing you learn, you have to find something unique in this book, which perhaps even the author was not entirely aware of. And that's what you hold on to, and that's what you add to the pictures: a whole Other Story that you believe in, that you think is there.
