It's been a long time coming getting to this part of the film. When I first started animating I knew that if I ever got to the point in the production where I needed to create the little guy above I would almost be finished. Well it's gotten to that point and I can start to see a dim light at the end of a long tunnel... it's kind of nice.
Anyways the second photo above contains some preliminary character sketches that I made before I began sculpting the head, the final version of which you can see in the first picture. I find it helpful to first sketch my ideas out before carrying it into the 3-dimensional medium of clay. Even if it's not final It helps remind me of what my original intent was for the character I'm creating. That way if I start to stray away from it while sculpting the drawings will always be there to keep me on track. Though sculpture definitely has a lot to offer in terms of designing a character. There may have been something you overlooked in your 2-dimensional drawing that has to be addressed in 3d that ultimately might end up strengthening the design. There's also a lot of little happy mistakes that contribute along the way as well.
Now I'm off to cast a mold for this fella's head since I'm planning on creating a foam latex version of it for animating.