Reviewing Common Mistakes
Many beginning cartoonists — and too many intermediate practitioners — tend to limit the emotional spectrum of their characters. Some find themselves concentrating so much on the mechanics and proportions of drawing that they disregard emotion. Still others fall into the habit of drawing the same figures the same way, assembly-line style. In either case, the biggest mistake in drawing emotion is failing to draw it in the first place.
Another mistake is rooted in character design. Often a character is designed with huge eyes that, even when half-lidded, tend to communicate shock and surprise. If you find yourself stretching the skull vertically to support the massive eyes you've designed, you may want to consider drawing the eyes smaller. It's like having a stereo that has two settings: loud and louder. You need to consider your character's quiet time, too.
Try to extend your emotional vocabulary by one expression a day. In your sketchbook, try to draw one face expressing an emotion you pick at random from a dictionary. Or try to draw an expression for each letter of the alphabet, once a day. Again, draw the face only. It will broaden your experience and help you to keep emotions in mind as you draw.
A third mistake is overusing a certain gesture or facial expression. For example, many manga-influenced comics have a tendency to overuse the wink. In American comics, it's the index finger sticking straight up to make a point. Both are very useful in expressing emotion. However, they can lose their potency if repeated over and over again.