Maybe he'll give us some insight into his process (hint, hint)?
Thanks. I've been learning to draw the human figure for many years (both digitally and on paper) and one insight is that features are rarely defined by lines, which can look cartoonish. For a more realistic appearance, it's better to use changes in value—light and shadow. So to add a jawline, it's best done by adding the
shadow that the jawline creates on the neck.
The Photoshop part is fairly simple. I first created a path to define the jawline.
I created a selection from the path and gave it a slight feather. Then, with a soft brush, I painted dark gray to create a shadow on his neck. Depending on the situation, I'll use a layer blend mode of Multiply, Color Burn or Linear Burn. In this case I used Linear Burn and layer opacity of 50%. Given the strong light coming from the left, the shadow would be most apparent just below the ear—where most people have a cleft—and then under the chin, which presumably juts out on somebody with a well-defined jaw. The middle area of the neck would have the least shadow because of the lighting, so I masked away 90% of that.
Dennis, I like the shadow you made, but in my opinion it's the wrong color and can be confused with the ruddiness of his skin. I think it should be closer to the color of the shadows on the dark side of his face (but not as dark because of the direct light).