So there's a thing I do. When I'm with people (people who haven't known me for 10+ years, or slept over my house, or get to be my boyfriend) I occasionally just agree to agree. I agree because I'm not sure what you're talking about, or because I don't feel like pursuing a subject, or because I'm just an agreeable girl. I feel like it's something most people do occasionally.
Over the weekend I was in a position to be with people I know, and like very much, but who were discussing someone else I know a little too negatively for my tastes. I was taken off guard because, in general, I run with a pretty non-judgmental crowd. And we all judge or say negative things occasionally, but still. I considered smiling, just to be polite, but I thought that might just make them think I was agreeing - which I wasn't. So I got up and walked away.
It may not seem like much, but one of my long-term goals is to be better with expressing myself when I disagree. It may not sound like this was a step in the right direction, but it was better than smiling and nodding. Maybe next time I'll experiment with wide/blinky uncomprehending eyes...
EDITED TO ADD: This is really not meant to imply that I've never said anything bad about anyone. I sure have. But I try not to. And now I will just try harder.