Since painting is done on a flat surface and the goal is to make your subject look three dimensional, it's important how your brushstrokes lay on the canvas. Using value changes gives flat objects form and so does temperature change -- warm colors come forward and cool colors recede. Hard and soft edges do the same thing, softer edges recede to the background and hard edges stick out and stay in front. Brush strokes can do the same thing. The video here
is an excerpt from my "Brushwork" instructional DVD. Strokes that follow the slope of a hill or mountain help in showing the shape and stokes that follow the shape of an arm or leg do the same thing. Long horizontal strokes make a distant field lay down and recede and contrasting short, choppy storkes in the foreground makes grass and shrubbery stand up and come forward. So think in terms of modeling with the brush.