There's research (though controversial) that suggests more than half of our communication with an other comes through body language. We instinctively know this, however, when dealing with difficult situations and conversations we can easily forget this.
Body language is just that - a language. Complete with a syntax and grammar. It is not enough to think that someone who crosses their arms is being defensive. We need to understand the whole "paragraph" they are speaking with their body. They may in fact be cold or they may just be leaning back and relaxing. We need to look at the whole body and its cue.