Lip lines, Nostrils, & Mouth Corners.

Below are examples that show what lip-lines, nostril and mouth corner details are and how they look.

Lip Lines - subtle lines added to the lips vertically to give texture.

Nostril - adding paint or blushing to the inside of the nostrils. May work for only certain sculpts.

Mouth corners - adding definition to the corners of the mouth to help with expression or detail. They can be added in a range of intensity from light to dark.