Wow. He can be pretty expressive. How comes you did him that fast and got him a wider range of emotions?
Short answer: I've been drawing emoticons so long, I'm comfortable with drawing different expressions. That was the easiest part. Making him lean forward, now THAT was a pain, especially when I decided he needed an inbetween image. Arrrrgh. Tweening is not fun when you're working with those kinds of images.
Long answer: Drawing the cels is the easy-to-explain part. Regarding the wider range of expressions, that's take some explantion of mascot configuration and programming. I doubt that anyone here really wants a long discussion of that - but in case someone does, I'd suggest looking at the tutorials on Otakuworld. They explain much better than I could.