Details matter in any quality endeavor, so the animation directors for The Simpsons provide specific instructions for how characters should be drawn and how they behave. From 1991 through 1993, the show published a fan magazine called Simpsons Illustrated. Among other features, the editors included some of these behind-the-scenes instructions.
For example, there were moral limits to what the main characters would do. One article stated plainly, “What is something a Simpsons character would never do? They never think evil thoughts.” That was probably a good decision. Other articles showed how characters are drawn, such as these guidelines for Lisa Simpson.