Quentin Blake fact told to me by art school tutor who knows him- when he does he illustrations, he draws loads and loads of quick versions of the same scene in different ways, cuts out all the best bits and makes a collage of them, and then traces that to make a final version.
(He told us this to not make people feel bad about using a lightbox - you get loads of ideas given to you at school that this is cheating, real drawings are done with soft pencils and shading etc, but actually a lot of professionals use stuff like lightboxes, propelling pencils etc)
