In watching back over the video of Sunday Game Group's Scion session, I realized one of the things that made it most fun is that the group is comfortable enough with each other that everyone relaxes into their characters and gets animated. There's lost of gesturing and reposturing to convey not only what the characters are saying, but what they're doing.
I think it's really an underrated piece of roleplaying to have the ability to mime out a bit of a character's body language. Posture and gestures tell us a lot and often help us interpret the words and tone of a person speaking to us. I think for SGG, those gestures help us remember that it's not Chuck over there, it's Nick. They might have similarities in posturing, but they're not the same and that can often clue me in if he's in or out of character.
No more point in this post than to observe that, but I do think it's really cool.