Don't be stuffy about it. If somebody wants to roleplay something silly or off-colour, let 'em. It's their role, not yours, it's not your place to judge.
Don't discourage social interaction skill rolls. This might sound counterproductive, but you must understand that some players just aren't very good at being smooth, manipulative or fearsome, nor at picking up subtle cues, just like most players would have a hard time lifting even their own body weight and may not know an aorta from an artichoke. The social interaction skills are there to help characters do what their players can't, and should be used as such. If you punish someone socially awkward for being socially awkward, he's going to clam up, so don't do that... let him use the tools the system provides him to be the suave James Bond type or ferocious mongol officer, if you want to encourage him to try.
Set an example. Try to provide a little more detail in what you're doing, and make it interactive. Nobody gives two shits about Generic NPC #27, even if she has a name. If she's clearly an elf, yet seems to be humming a harsh orc tune on her way to market, pausing only to scold and shout at children playing in the street, that's distinctive, and might draw attention. Be ready with the how and why of it if the PCs check it out -- maybe she sympathizes with the half-orc underclass (and can point them to a short side-quest involving them) because she feels the higher classes in the city have gotten too lazy and helpless, and dislikes seeing children playing when they should be working apprenticeships -- and don't bite their heads off for it, even if she is snappish... remember you're trying to encourage the PCs to interact, and you'll do exactly the opposite if they get the impression that it's not safe because the first thing they touched exploded.
