Lines can express everything from value to form to shape, especially when you judiciously wet your paper. When you add in the water, touch it only here and there—just where you want to suggest shadow or volume.
You also can use a combination of techniques for variety. Mix narrow lines with broad, washy areas. Add repeated crosshatched lines into the shadows if you like. If you’ve lost too much definition once the water has dried, go back and restate where you need a bolder line. Or you can add the restatement while the paper is still damp to give you a very intense effect. Be advised though—if you go back in while the paper is still wet, you’ll have a harder time later altering that look if you’re not happy with the result.
Rather than filling spaces with solid tones, draw lines with your watercolor pencils as you would with pen and ink.