Now that 99% of the drawing is in place, it's time to refine the lines and add paint. Until this point, the paints are applied in thin washes, for several reasons. It makes dragging the brush across the canvas, and still applying paint, easier. Dry paint doesn't go very far. Thin paint doesn't cover much, so it's a delicate balance.
Painters follow a principle of "thick over thin," or "fat over lean." This is important because thicker layers take longer to dry, and will cause cracking if the top layers dry faster than lower layers. This shows you just how flat I work:
This is the best stage of the painting. I can focus on the volume of the subject matter by adding shadows and highlights. I can pay attention to finer details and bring life to the canvas. I want to create the illusion of depth. I want the viewer to feel like they're in that place at that time. This illusion is what drives me to paint.