Secondary Emotional Indicators
The mouth, the eyes, and the eyebrows are primary indicators of mood. Respectively, they control the degree of pleasure, intensity, and aggression that come together to compose an expression. However, they cannot exclusively complete a full spectrum of facial expressions. Secondary emotional indicators are used to focus these expressions further until they communicate specific moods. The most frequently used secondary indicators are teeth, eye bags, and wrinkles.
Many facial features follow an “appear as needed” rule. When they're needed for an expression, they appear. Otherwise, they are not drawn. All secondary indicators fall into this category unless they are a specific part of the character design. As stated earlier, the mouth and eyebrows also follow this rule.
Showing Teeth
“Baring one's fangs” at someone is a sign of aggression that goes back to early mankind. Consequently, adding teeth to an expression can elicit a visceral response from your reader. Any expression that shows your character's teeth becomes intensified — more hard edged.
Eye Bags
Eye bags appear when the face is expressing illness or sleepiness. On a smiling face, they undermine the sincerity of the happiness being expressed. Crow's feet can often accompany eye bags. Of course, both of these features can also be used as part of the character design to indicate an elderly character. However, if you choose to go that route, be aware of the obvious effects it will have on all of the facial expressions your character forms.
Wrinkles
Cheek creases are the lines that form on either side of the mouth, extending from the sides of the nose to the sides of the mouth. These lines appear only if the mouth is being stretched or shaped in a rigid fashion. Like eyebrows, the appearance of cheek creases intensifies an expression.
Forehead wrinkles appear when the character is worried or ill. They aren't necessarily straight. Most of the time, they follow the expression displayed by the eyebrows. However, they're not bound by physical law. They can become wavy or choppy, depending on the emotional demands of the face.
It's important to familiarize yourself with all of the facial features and how they help build an overall expression. However, it can be just as helpful to simply hang a mirror in your studio. This can be useful for illustrating complex emotions or ones that don't have a clear structure.
