Today I went back to one of my timed sketches of Callum and made some improvements and revisions. Mostly I added shadow and definition where it belongs, smoothed out a few lines, etc. Here's the side-by-side results:

I think in particular I could profitably do some work on his face. The shading there looks pretty raccoonish, which was not at all my intent. His torso could use a little bit more work, too. 

I'm do like the new three-dimensionality I managed to get by adding in just a little shading, especially on his  legs and his boots.