When I begin to paint a representational work, a major factor in my mind is reality and believeability. Three dimensionality is an important component in this and proper use of value is the most important factor in creating the appearence of three dimensionality.
Contrasting values create depth. Only against the very dark values do the light ones shine. If there is no dark, the light fades and the depth weakens. Large simple patterns of contrasting values will improve everything about a painting, color, line, detail, all will pop and the resulting design will dazzle. As I paint I am always thinking about which shapes belong to which values and therefore what sequence to paint them in. When working in transparent watercolor, it is necessary to pay attention to the water and pigment and how they move on the paper, once the white is gone, it isn't easily retrievable, so the sequence must be planned like a dance is choreographed.
Color is also important to the illusion of a third dimension. I love working with color. There is a chemistry which happens between colors. The juxtiposition of complimentary colors creates a vibration which makes the area stand forward and sing. Glazing, the building up of multiple layers of transparent color, can also help to create the perception of contour and depth.
Believability does not depend only on three dimensionality; reality also has rhythm, patterns of shape, line, color, and value which repeat and build on each other. One of the joys of painting for me is trying to understand and capture the rhythm which is special to a particuar subject, to get the viewers eye to dance along its singular path of vision.