Body language is a powerful means of communication because most human communication is nonverbal. We meet another person's eyes, or we look away. When we're nervous we tap our fingers, bounce our legs, or fidget with nearby objects. Our tone of voice supports what we're saying or contradicts it.
Weaving the details of nonverbal communication into dialogue adds texture to the conversation and provides fodder for action bumps, helping to weed out attribution tags. Such details also help to establish the mood, which cuts down on the dialogue tricks you need to communicate with the reader.
Nonverbal cues can deepen the characterization, as well. Refusing to shake someone's hand or return a gesture of friendship when one is expected, puts the other person off balance. They'll react according to their own nature, but the reader will get hints at each of their personalities and their relationship. It makes a good tell.
But how well do these cues come through in the reading? We're used to seeing or hearing them, so how do they translate to the written, or read, word? After all, no matter how many nonverbal cues we work into a story, they're just extra words if what we're trying to communicate doesn't come through to the reader.
Some nonverbal cues come through just fine. For instance, someone slamming their first down on a table. It's something we've seen often, possibly done ourselves. It's emphatic and there's no doubt as to its meaning, even if it may require further dialogue or cues to communicate the exact emotions.
The subtler cues can get overlooked, but I don't think that matters. Even if they don't stay in the mind, I think they contribute to the reader's impression of the mood, the level of tension, etc. They have a similar impact to the words that we choose. Using verbs that work with the context of the action they're describing helps to build an image for the reader. I think that subtle nonverbal cues do the same.
But because they are subtle, they're more likely to be dismissed, or to fail to communicate their point. Knowing when to use larger cues and when the subtle will be more useful sometimes requires tinkering and critique or reader feedback, but switching between the subtle and the not so subtle keeps things from becoming stiff or overwrought.
Of course, too many action tags and too much body language can lead to the characters hopping around in their seats trying to convey things to the reader. As with everything, it's about striking that balance. Critique really helps with that, too!
So how do you deal with body language when you're writing? How much do you describe and how much do you leave to the imagination?