A couple of years ago I ran into a trainer who worked with dolphins. As we talked about his work, he mentioned that you have to be very careful when working with them not to reinforce the wrong behaviors. The reason is because they are incredible perceptive and notice everything. Often they would notice a pattern that he wouldn't even be aware was in place. When the dolphins would detect a pattern that reinforced something he wasn't aware of, they would use it to their own advantage. The key to their perceptiveness is peripheral vision.
Focal vision is the central element in your visual landscape. It is primarily what you use to distinguish details or read. Your focal vision is very much a conscious activity. On the other hand, peripheral vision is wide, fuzzy, and happens subconsciously. Your peripheral vision is excellent for detecting movement and establishing a frame of reference.
You can use your peripheral vision in many situations to obtain excellent information about other people. It is the primary way to easily absorb specifics such as breathing patterns, gestures, and eye movements. These are all key things you would want to be mindful of as you try to build rapport.
It's pretty easy to practice using your peripheral vision. When you are in any low-risk situation, use your peripheral vision to see what else you notice during the conversation. When you are working with groups of people, you can notice what’s going on for all the people you are not looking at directly. You can practice reading the signals, gestures, and expressions being made by the people who think you can’t see them. You can learn a lot while practicing this at times when you are not the primary speaker.