On the disgruntled sergeant he plays in Halo 3: ODST, Serentiy actor Nathan Fillion volunteers the following in an interview with MSNBC:

There's a former romantic interest who's been thrust back into his life on this mission. He's in a bit of a high-stress situation. So what might have been a more pleasant war-time demeanor is now a more stressed-out cranky war-time demeanor.

From the footage, it looks like Bungie is also licensing Fillion's likeness. Good going! If you're going to get a celebrity to voice your character in a videogame, you might as well go all the way and get the celebrity to (virtually) play the character as well.

Halo 3: ODST is a stand-alone expansion due this fall that will follow a team of orbital drop shock troopers played by Nathan Fillion, Tricia Helfer, and you, the player.