I usually stand there with a blank expression, but not on purpose I'm just processing the moment. When I finally do react, I'm usually not very aggressive about it. I will tell them to chill out and what there mad about is not really a big deal, and if they are still mad at me I leave. I'm not the greatest at solving disagreements, if you can't tell.
But if I'm in a terrible mood already, I can get emotional. But that involves me leaving pretty soon as well.