I did a live demo, designing a bad guy for a fictitious game at CDWA this year in Adelaide. It was great fun and I promised I'd post the WIP with some notes...
After drawing some rough b/w thumbs based on triangles (they attract attention) and a heavy-set figure (looks brutal and hard to kill) I selected three (pic1) and narrowed them down to one, where I refined the outfit (Pic 2).
After making a basic pass at some poses to describe his energy and brutality (pic.3) I settled on a lumbering run (to give the design some character) and quickly blocked in some colour values and refined the pose (pic.4). I wanted him to be cold in contrast with the warmer toned hero from the previous class.
Finally (pic.5) he's worked up a bit with some final lighting and his weapon for detail. All up about 8 hrs from scratch.