The expression on a cartoon face is a combination of the eyes nose and mouth, and any lines drawn around them to emphasize the expression. The four drawings below for example show the same basic egg shaped cartoon faces with the same ears nose and hair. However changing and/or adding lines around the eyes and mouth can completely change the expression on this man's face.
I often leave the hair as an afterthought which is unfortunate as a character's hair is an important factor when creating a cartoon. I often leave the characters bald or with little hair, or prefer a simple head shape with a few strands of hair on top or above the ears to give the cartoon character, rather than creating a detailed haircut.