Another non verbal response when communicating with someone is lower lip changes. […] These are nearly impossible to control. Changes in color, texture, size, firmness, trembling etc [of the lip]. This requires some practice, but if you start paying attention and take mental snap shots, you’ll soon begin to notice these changes, and what they mean to a specific person.
Great example of subtle calibration cues! It points to all other apparently minor cues you can pay attention to when eliciting or testing a response.