Each time a new thought passes through your sitter's mind his or her mood changes, and the expression around the mouth changes as well. Anger, boredom, fatigue, disdain, petulance - all these feelings and a million more show around the mouth. Don't go back to the mouth for any reason. The lip tissue is darker in value and warmer in color than the surrounding skin. When the mouth is open and the lips are parted, we see the teeth.
Infants have very full, soft, rounded lips.
In old age, the lips become thinner. The upper lip is often in shadow, as it is a receding plane. The lower lip catches the light, as it is a projecting plane. The mouth is the most challenging part of the portrait for the artist because it is constantly changing, but the changes are usually extremely subtle. When drawing the mouth, remember that the upper lip is made up of three parts, the lower lip of two.