When reading a book, we are often more focused on what the character is saying than on how their mouth appears, but the way in which a character's jawline and mouth is described can round out the image the reader has of him or her.
A jawline can be set and rigid, firm and unyielding, square and determined.
Or it can be soft, angular, receding...
Or fleshy, sharp, covered in hair, or waggling.
Lips can be thick, voluptuous, puffy, like pillows, or swollen...
Or they can be thin, nearly invisible or non-existent, pale, weak, set and straight...
The lines around a character's mouth can show a lifetime of laughter or of sadness; they can be down-turned, deep, dimpled, upturned...
And they can be red or pink, anemic-white or pale, chapped, smooth, silky or rough...
A character's mouth can draw you in or push you away, and it can change your perception of them from one scene to the next.