There is subtlety in a mouth that is hard to capture. A simple smiley face captures the minimum needed to express the emotion, but lacks conveying anything more deep. I tried today to draw the important parts of the face, but was let down by my work. All the eyes I drew were (on accident) very angry or surprised. I had trouble expressing gladness (as opposed to happiness), being bored, being let down, and excitement. The pupil size is the easiest to change, and the eyebrow (muscles, bones, and all) seem to be the hardest.
The mouth is also complicated, usually resulting in a face that looks bizarre or rubbery. Looking at real faces, the mouth can be as wide as the outer edge of the eyes, or very narrow. The dimples are also puzzling to me, with inward dimples looking pudgy, and outward dimples looking wrong. I am sure I have seen the dimples drawn on faces before and look correct. Even the distance down the face was hard to figure out. Putting the smile too low made for long faces, that appeared either sad or malevolent. A smile too high up looked again like a pudgy or squished face.
Here is my conditioning drawing (cubes, cylinders, and an attempt at a padded chair) as well as my attempts at eyes and mouths: