If it's a first person game I'll tend to make facial expressions of what I imagine my character would be doing depending on the situation. Take Half-life 2 for example, look mad as I beat a metro cop to death with my crowbar, look worried as the teleporter buggers up, gape slack jawed when Alyx first enters the room...and you see that sweet automatic pistol of hers in action. If I'm playing co-op with a mate and the game becomes really immersive, I'll act like one of those war heros in the movies. You know, the kind who always say things like "I'll make a dash to the right, you lay down some cover fire" as they lie next to their friend in a trench being assaulted by bullets.