Industrial psychologists and others can give you statistics on verbal and non-verbal communication, but I have been led to believe that as much as 60% of communication is body language. That seems a little high but I can tell you from experience it's pretty close to the mark.
In my experience body language can shift a conversation much more quickly than the spoken word. Here are some examples and how they might be used.
Symmetry is basically your position to the person you are communicating with. Let's say you and the other person are sitting in chairs. They are facing toward you, but you are facing 45 degrees from them. Your symmetry is off. If your head is cocked to one side, or one side of your body does not match the other then the symmetry is off.
By shifting to face the other person and leaning forward with an open posture you can indicate acceptance and interest and encourage the other person to follow this line of conversation. By turning away and closing your posture you can get them to shift the focus and communicate your lack of interest in this line of communication.
Some executives use height to demonstrate their dominant position buy raising their chair higher while making the chairs on the other side of the desk lower. This is wrong because during open communications you want to be at the same height as the person with whom you are communicating.
I frequently use height to politely end a conversation. We have all worked with someone who frequently interrupts and drones on. When they entered my office and sat down, I would stand up. This generally ended things pretty quickly so I could get on with my day.
Posture can convey interest or disinterest. You can slouch or sit up straight. Each of these actions conveys something to the other person.
Open and closed positions
If you fold your arms across your chest it is generally viewed as a closed posture. You are communicating that you are not listening. Arms at your sides communicate a willingness to listen.
I love this one. Many of you that are parents use this on a regular basis. One of my children will ask a question or make a statement and they can read my face like a book. A scowl, squinting the eyes in an "I don't understand" way or just shaking the head can convey much more than the spoken word.
I have heard people coach on body language. They say sit up straight to maintain an open position and use positive body language always. I think that's like signalling a landing aircraft on a carrier with all positive signals. Bad idea. Sometime you need them to do something different. So the next time you speak with someone, use the information here and gauge the response. You might be surprised.