Since this is my question, it's about time I answer it!
That said, how do you keep the mood? And once lost, how do you try to bring everyone back? Can you? Is it even possible?And what do you do with that one player who is always the first one to crack a joke and break up the tension you've built to so carefully, no matter how many times you've asked/warned him/her not to do that?
I had started to answer this when I submitted the question to Ginger, and had to pull back, to save it for the reply.
Back in my very first Amber game as a GM, one I called Prime, we had a specific thing that we did to get people back on track. I don't know how it got started, but one of us, who noticed that we weren't gaming anymore, started humming (well, more like doing "da da's") the Christmas carol, Good King Wenceslas, and everyone would take notice, some would join in, and then we'd get back on track. Until the next distraction.
As that gaming group morphed into a new one, with some leaving and some staying, the original song was replaced, in favor of the scene change bit from the end of Wayne's World, the little doo-doodle-doodle-doo's complete with fingers waving as you moved your hands down. This hasn't been as successful, but it is still used.
I don't really have a good answer to this question, which is one reason why I posed it. As a GM, I understand the need for breaks and levity, but I also want to keep a mood going, to help convey the setting I'm trying to get across. Most of the time, I let the distraction go. Frell, some times I join in, and the players need to remind me that we're supposed to be gaming.
But that never helps when the time comes where I try to get deep into the mood and character (if I'm playing an NPC), and the mood is shot by an off-hand comment, and I get upset. It's a bad habit. I know we're not kids, but consistency works with gamers too.
There was one game where Lou ended up cancelling the rest of the session because he was so distracted and not on task, and the OOC talk was the main factor. There's sometimes where a "Cut!" is needed, and maybe you have to return next week/two weeks/month in order to restore it.
I'm trying more now to talk quietly for some NPCs, to make the players strain to listen to them, so that they have to pay attention. But I don't want to be known as the Whispering GM. I think that different voices/gestures/postures for different NPCs can make players sit up and take notice, though some might be more likely to poke fun OOC than to go with it.
With those folk, I think you just have to give them a certain number of warnings. Then you have to agree to disagree and part as GM/player, so that you can hopefully remain friends. It's tough, but I think in the end it can make the situation better for everyone.
As a player, I'm pretty good at staying on task and on track. I'm one of those gamers who's in it for the role-playing, though the socializing is fun too. And if the GM takes me to task for cracking too many jokes, then I'll agree (as long as I'm not singled out if I haven't been the only one making the jokes...there's nothing that'll get my back up faster than someone choosing to use me as the example) and try to be better.
There are no easy answers to this question. But I have seen some interesting ones - some I'd never use, but now I know what I'd never try.