There are actually people like Cal in real life. He studies emotions. In order to detect lies he has to know what triggers certain emotions and that the fuse is slightly different in different people. However, he can read them, he can see how they respond to different things and thus guage what might be needed to trigger the response he is looking for. He can also present the appearance of those emotions to other people, even if he is not initially feeling it himself, such as the attack on the girl with DID.
You often see him in the first series echoing people's positions (I am thinking of the child who kept his friends backpack in the kidnapping episode) as well as their emotional state, how often does he say "I understand how you feel" (Gillian also does this)? This is to get people to relax and open up around him, people will volunteer more information using this technique. He also uses agression to trigger certain responses, to get people defensive and likely to give up the truth (again used in many episodes, including Moral Waiver and Blinded). He manipulates and coerces people into doing what he needs them to do to reveal the truth. He uses his body and other people's perception of him to excellent effect. It is far more evident when he is being antagonistic than when he is being nice, but the latter is there also.
How people percieve him is the control he has in each of these situations and a control everyone has but that few people realise.