Definitely a nice article. Our body language, our facial expression and the way of talking change with the state of our mental condition and emotional state. Also, much depends on the personality of the person who is being questioned. Some people easily frightened and some are more robust and tough. So, before testing the body language and other non-verbal characteristics of a person, his/her emotional background and the mental state should also be determined. Otherwise, it may happen that an innocent but nervous fellow may be termed as liar because his nervousness prompted the examiner to believe that he was lying.
In opposite to it tough people can hide their true emotion by showing artificial expression and demonstrating confidence while being interrogated. Many hardcore criminals are master of this art. They can simply switch their mind according to the situation. If they know all about the body language and non-verbal communication, they can manipulate the things for their own advantage.
It also depends on the person who interrogates. If his intention is malicious, he would ask those questions which may lead benefit to the accused or vice-versa. He can also make an innocent person a scapegoat in this situation.
It is also true that many persons cannot understand non-verbal communication. I have seen many who don't understand what your body language is telling.
Therefore, it can be sad that this science or art, whatever it may be, is not free from errors. Chances of misinformation are always high. On accounts of this reason many courts don't count the results of these investigations as an evidence. In spite of this, body language and non-verbal communication tells much about a human's characteristics and his emotional, mental level. More study is needed in this field so that its results may become reliable.