When drawing from observation you can add interest to your image by using different kinds of marks. Look at the amount of detail and information this drawing provides while also showing the action of drawing.
Different sizes of hatched lines can add both action and value.
These vertical marks are a nice textured difference from the blank background.
This drawing was made by first drawing the bird in flight, erasing the first marks and drawing on top of the first drawing. Notice how much motion this adds to the piece.
William Kentridge makes very exciting animations using expressive marks to draw the subjects and then smudging and moving the lines. Even this still image from the animation is exciting.
The marks in this drawing are achieved by drawing with charcoal, smearing, and erasing.
The marks in this Salvador Dali drawing suggest motion created by the different lengths of lines. The action of drawing is evident in his final piece.
Using expressive marks in your drawing does not mean you have to loose control of your materials. The controlled hatching on this drawing shows astounding amounts of detail while still expressing motion.
This beautiful value drawing is made up of small marks. In this piece it creates a sunny, soft feeling.
Different marks in this drawing define different things. Notice the round, sweeping marks suggesting the light radiating from the night-light.
The active marks in this drawing add interest without distracting from the realism and unusual viewpoint.
In this similar drawing the active marks have been toned down by drawing over and over the lines, still showing the action of drawing but allowing for a more refined finished surface.