He's actually a very well trained actor. I think he got stuck in a lot of little comedic roles early on and got typecast there after since he was just so damn good at it. Which is a pity, IMHO, since he's *expletive deleted*ing brilliant.
As much as I love Wash and Steve, my favorite role of his is Alpha in Dollhouse. Mostly because Alpha is like 50 different personalities in one package and most of them are completely psychotic.
"There are many parts of me that know this is wrong. None of them care. And six... they just find it funny."