When you watch other people you don’t always need to be told what their relationship is with each other or what that relationship is like — you can see it in their body language, their eyes, and hear it in the way they speak to each other.
Using descriptions of body language in your story shows your reader what is going on between two characters rather than telling them. It helps to create an emotional response in your reader, which reflects the emotional responses your characters are experiencing.
Check out some of the books you’ve been reading lately (you have been reading, haven’t you?) and see how your favourite authors manage to convey relationships.
Tension or friendship or love between two people shows itself in many ways: how close people stand to each other; whether they are turned towards each other or away; little touches on the arm or hands when people are in a close relationship; whether faces are relaxed and smiling or taut and expressionless.
Observation of body language is important. Watch people the next time you are in a restaurant, a shop, out in the streets, or just at home or with friends. What does their body language tell you about them?