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After understanding everything about your characters, the next step is determining how to reflect this character in computer graphics. Like the little smirk in the story above, you should find signs that can reflect the inner emotion.
Figure 3: The woman below has been given a stretched mouth to reflect her deeper character.
See Figure 3 for an example of these subtleties. The image on top is a typical mouth shape to be used in production -- easy to rig and weight.
The image on the bottom has some good subtlety that shows her personality and history. She was not happy with her small lips, and always tried to make them wider. It became natural after years of trying.
As another example (Figure 4), eyes change when a person focuses intently on an object.
Probably the muscles around the eyes tighten so they can adjust the focus -- there is a huge change of impression when the eyes focus, even though there's not much change in shape.
Figure 4 shows the difference in expression when the eye focuses intently on an object.
It's now the era of ZBrush. Especially in the game industry, details of 3D models are increasingly crafted in ZBrush, so the flow and edges of a surface can be modeled like clay, using a polygon structure. (I would not like to call this an edge loop, because the concept for edge loops deals with subdivision and tessellation, whereas polygon structure is meant for flows like this.)
An edge needs to go along a contour, and since one polygon can only share four edges, it's a challenge like a puzzle. Ultimately you need to pick some edges and throw some away to maintain the quad polygons.
I would say this is the most painful process and takes the most time when creating a 3D model, but this is how good models are made. Lots of time is spent on these areas that are seldom seen by non-professional eyes. ZBrush or Mudbox will release you from some of this pain, and four-sided quad polygon limitations (see Figures 5a and 5b).
Figure 5a: Detecting flow makes objects easier to carve.
Figure 5b: Flow changes depending on the person, and even changes when muscles are flexed.
With these software packages, the polygon structure does not have as large a role as it used to, but still cannot be ignored. Outside of games or in some situations like softbody simulation, you can't get away from it.