Shading

I was wondering how does professionals do shading on their characters. I was watching the bonus features on iron giant and it shows a scene with hogarth and theirs like this blob around him and that blob ends up getting filled with the shading i guess but i want to know how they do this. Do they scann the shading seperatly or is it on the same piece of paper as the character. I tryed drawing the shading on a character by hand but when i import it into toomboom it is reconized as one line the character and the shading. any links or information on how shading is done will be appreicated.
Thanks

In traditional animation shading was usually added on an additional cel layer above the main art work. So a shaded character used two cels per image. This was very limiting as in photographic animation the number of cel layers used creates optical problems in how colors are seen. Paint would have to be mixed differently depending on how deep in the pile of cels the color was positioned. This of course isn’t an issue in today’s computer rendered animation. So shading can be done in several ways. It can be directly incorporated in a single drawn element during the paint process using stroke found under the paint tools as the means of producing the outline of the shading in the drawing. Stroke is a very useful tool as it creates a fill zone outline, which doesn’t show, but can be filled with a color that has a small alpha value so that it is translucent.
Additionally shading could be done in a separate element layered on top of the main character element. This is often the best choice when you want to keep the main character animation for later reuse without this shading. And thirdly you can use a color transform effect as a shading technique. This is more complicated but can be the most effective approach. -JK

How do they do it it movies today like iron giant or lilo and stitch, movies like these.