Body language is as important in writing as it is in real life. If you don’t want to explain what your character is feeling or doing, the best way to show it is by their body language.
For example, a person who is lying will often rub an eye, touch gently beneath an eye, put a hand up to the mouth, touch the nose or lips with a finger. Sometimes a liar will tug at an ear, scratch the neck, pull at his collar, jiggle a foot, blink more than normal. People tend not to look in the eyes of the person being lied to. And they hide their palms.
Showing the palms is a way of saying that someone has nothing to hide, and it is a gesture that we all subconsciously react to. But the gesture can be faked, and so can looking someone in the eyes. If you want to show that a character is lying, you can have your liar look another character in the eyes while showing the palms. By mentioning these two signals together, they become important, and can show that the first character is purposely lying. Of course, they can also show what they normally do, that the character is telling the truth.
That’s the problem with body language: it is ambiguous. Scratching the head can mean that a person is thinking or is confused, but it can also mean the person has dandruff. Rubbing an eye or covering the mouth can show that a person is hiding something, but that thing can be as innocent as hiding sleepiness or a yawn. Folding the arms across the chest can mean a person is defensive; it can also mean the person is cold.
Certain signals are subtle in real life, but when used in a scene, they can telegraph the truth. A person generally crosses their legs toward a person they like and away from a person they are not interested in. If you write that a character smiled and crossed her legs away from him, it’s obvious what is going on, even though in life we seldom catch on as quickly.
The same is true of pointing a foot toward the door. It’s a signal that the person wants to leave, and that seldom-noticed signal becomes obvious when written.
Another bit of body language that works well in print is mirroring. A subordinate mirrors the body language of a leader, so your band of characters might have a nominal leader, but their true leader is the one they ape. Interestingly, team members who works well together have the same posture and body language, which shows the rapport of the group.
The best way of learning body language is simply to watch people. Look at their hands and feet when they talk. Have friends purposely lie to you and see how they act when they do. Pay attention to your own gestures, and try to keep them at a minimum. Not only will you be harder to read, the fewer the gestures, the more intelligent and refined you will seem.