Most
miniature painting uses some concept of a directional
light source in the process. Some model painters
refer to this concept as the "Halo of Light,"
a circular ring of illumination that you would place
above the head of the model. You can use this imaginary
halo to figure out where light will fall, what extreme
highlights will occur, and what shadows will lurk
in the recesses. The halo can even be moved around
the model to create interesting forced lighting
arrangements. That technique works very well in
dioramas, but it tends to look strange in normal
gaming circumstances where the models are being
moved across the tabletop.
An easy way to figure out how light falls on a
model is to hold it under a strong light source
such as a table lamp. It will allow you to see exactly
how the light covers the upper portions of the model
and how shadows pool in the recesses. You can then
reproduce what you see happening on the model itself.