OK, I've got one for you. Here's another one of those little-appreciated tools of expression. Well, not so much little-appreciated, as rarely-verbalized: the quality of the edges of shadows. Not cast shadows, but regions where the turning of a form takes it from lit to unlit. In this case, we will compare the arms in two of my paintings, because the comparison illustrates my point fairly clearly:
The left painting is that Emma painting I finished recently. The right painting I am working on now, with Cassandra (actually, let me throw in a plug for her fantastic dance company, Desert Sin, as long as we're talking about her).
Now, the light sources in the two paintings are actually the same - I'm using my 500-watt tungsten softbox raised on a C-stand to a height of about 7 feet. The diffusion is the same for each painting. The models have similarly lean, muscular arms. But the images, obviously, look very different.
Apart from the choices I made in terms of color and value contrast, the quality of the edge of the shadow is different in each case. For the Emma painting, I wanted to produce a sense of simplicity, starkness, and stripping down of appearances to essentials. So I made the shadow edge artificially hard and stylized. In the upper arm, much of it is virtually a straight line.
This comes across as crude, childish - but I wanted it to; I wanted to avoid the trappings of skill, which slip so easily into tricks.
For the Cassandra painting, I want a sense of light and floating. And I have allowed this to inform the shadow edge on her arm as well. This shadow edge occurs only on the forearm. I have followed the actual curve of the edge much more closely than I did in the Emma painting, and at each point along its extent, I have asked, and tried to answer, the question: how hard is this edge? Often it subsides into soft indistinctness. But sometimes it is hard, where it is near the joints of the wrist and elbow.
In cinematography, a very simple version of the dichotomy illustrated here is always addressed: is the lighting hard (as in the Emma painting) or soft (as in the Cassandra painting). The issue is rarely pushed much farther than this, because film involves coordinating so many elements that people really don't have the time to screw around with lighting as fancy as the resolution limits of film allow.
In painting, there is much less to worry about, and painting isn't photographic anyway. There's plenty of time to resolve and stylize the most sophisticated analysis of the hard/soft distinction over every part of the painting (if you go back and look at the Emma painting, you'll see it's not hard everywhere). Be that as it may, the edges of shadows are powerful expressive tools, and one ought always to consider how the depiction of these edges integrates aesthetically and thematically with the painting overall.