Emphasize those reactions without seeming too self-aware. We have bodies. Those bodies move. Such details lend a sense of placement to the piece, and help the reader visualize what you are expressing. Focus on the detail, not the context.
Individual mannerisms are essential for character development.
All these little details can help express the sort of person they are. I know a man who pulls his own fingers when he's nervous. People don't use dialogue enough. More often than not, dialogue will take care of all your character development for you, if it's recorded accurately. Of course, remembering who said what is the clincher.