Good man for doing these, I should take up a similar task. I have an extra challenge for you in doing these. Think of everything about the character before you start, like the body type, age, pose, clothes/weapons, expression. I think itl be more beneficial if you do it that way. But obviously you dont have to stick to what you think of at the start, its just a starting point.
Funny that you mention it because I actually did with this one! Gotta think more though, but sometimes I just have half an hour, in which case I just get straight to doodling whatever the fuck comes to mind :P