Writers have back story, and I find it works for telling a cartoon story as well. As I continue fleshing out the character, I go over who this character is really. BoxJam is lazy, he loves TV, he likes beer, and he has a childlike appreciation of his world. Aha! It's this sort of exercise that led me to put a hint of a teardrop in his right eye.

I also think now about the lighting source - in this case, the blue sun - and mark on the outline the dividing line between the parts of the body that receive direct blue light, and those that don't.