Step-by-step: Paint a range of facial expressions
The zygomaticus minor muscle curves the mouth downwards and the corrugator supercilii pushes the inner parts of the brows upwards, giving to the face a sad look. In a normal face this would cause deep skin folds on the sides of the mouth, but in a skull-like face, I minimise them. Less fat under the skin results in smaller folds. The procerus muscle lowers the inner part of the brows. Combined with the caninus muscle and triangularis muscle, which raises the inner part of the lips, it also helps to flare the nostrils. This creates a look of anger. The foldings on the mouth and brows are inevitable, but vertical foldings on the lips can enhance the thin look. The zygomaticus major muscle raises the edges of the mouth, creating a cheerful expression. The occipitofrontalis muscle raises the brows and the orbicularis oculi muscle lifts the lower lids. I paint some horizontal skin folds over the brows, and make some muscles more visible under the skin to make him look thinner.