RPGnet forum regular Ian Noble describes how this came to pass...
So it's Saturday afternoon at GenCon SoCal this past weekend and my Gurps Firefly game I'm running seems to be going quite well, the adventure is building toward a slam-bang finale and everyone's doing great with their characters, the roles from the show.
Then it all gets surreal when Nathan Fillion, the actor who plays Captain Malcolm Reynolds in the series, shows up and sits down to play.
Turns out he had been signing autographs elsewhere on the showfloor and one of my players very nicely asked if he'd stop by. For all his awesomeness, he did.
An incredibly nice guy, we clued him in on the scenario, went over the character sheets with him -- he got a crack out of some of the quirks I had written up for the "Mal" character -- and he rolled some dice with us.
... He mostly just hung with us for a while but for a few, brief moments he tried his hand at playing River, an NPC in the game. No, I'm not joking.
But, honestly, it's not like any of us could actually *concentrate* on the game what with others running up and camera flashbulbs going off...