This was a memory sketch from a lunchbreak, several weeks back. I pick someone out that looks interesting and observe them for a few minutes (I don't actually draw at that point) and then when I get back to my desk, I try to remember what I saw.
These were the initial drawings I did when I got back:
I drew each of these in a matter of seconds, trying to get down everything I could still recall of the guy. These ideas then got developed into the drawing at the top.
Memory sketching is a fun exercise in character design that I really should do more often.