I was flicking through 'The Art of Storyboarding' and found these few pages helpful.
This page talks about how to draw the characters. It basically echos what Phil said, which is to simply roughly sketch them out. Start with stick men then work in more detail by adding circles and other shapes to give them a 3D appearance.These 3 pages were just some completed storyboards. The middle two show the use of the direction arrows.