The author says that being shifty-eyed and stammering doesn't, contrary to common belief, always mean that someone is lying. When people are lying, they tend to change their usual mannerisms with their face, hands, and voice, but you have to get a baseline to know what is usual for that particular person.
He brings in Elisabeth Kübler-Ross's schema for the stages of grief – acceptance, bargaining, anger, depression, and denial – and says that it really applies more broadly to how people deal with stress. Only in acceptance do we "take responsibility for our actions." In the other four attitudes, we are trying to change or hide some part of the truth. When interrogating someone about a factual question, if their response can be classified as bargaining, anger, or denial, they might be lying, and if they exhibit depression, it might "complicate the process of getting to the truth due to the layers of emotional pain involved."