By Helen South
In this 3/4 view, you can clearly see the effect of foreshortening on the curve of the lips, on the right of the photo the line of the mouth is almost parallel to the picture plane, while on the left it drops away, almost perpendicular to the picture plane. The amount of curvature depends, of course, on the individual, as well as their expression - some expressions pull the lips more tightly around the teeth.
Here you can see that there is very little change of plane between the edges of the mouth and the skin - the lips curve in gradually - though it is a little sharper along the edge of the upper lip. (This can be more obvious in different light.) There is a slight crease below the lip, where the shadow drops off, before the outward curve of the chin begins to catch the light.
You can also see how the skin gradually begins to crease at the corners of the mouth - sometimes this crease will create a quite obvious shadow.