Non-verbal communication, also known as "body language", or proxemic behavior, is the combination of inadvertent and innate body reactions that surface automatically when people intend to convey or add additional meaning to a verbal message.
In a court of law, these proxemics are observed as part of a testimony because, according to anthropologist Ed Hall (1966), the innate tendency for all human beings to communicate through bodily expression serves almost the exact same purpose as communicating verbally.
Proxemic behavior is the proper anthropological name given to non-verbal communication and it is divided into eight categories in which posture, personal space, kinesthetic reactions, visual codes, and the tone of voice are included.
Hence, just like people can discern an unknown language by following the physical expressions that go with it, a jury can discern quite easily whether a person giving testimony is using their innate tendencies for proxemic behavior to either convey a message or to deny it; to tell the truth, or to lie.
This is the reason why many police districts employ the services of body language experts: to create a baseline of immediate behaviors that surface naturally from a witness's proxemic behavior. This helps to create a profile for witnesses who are potentially telling the truth, and those who are not.
Take, for example, the information given in the Tolmage, Peskin, Harris, Falick information page and look at how body language is so important that even attorneys address it, prior to representing a potential client:
It is impossible for an individual not to communicate, even if he chooses to remain silent.
It is generally agreed by experts in the field that over 60% of the impact of meaning of the communicated message resides in the non-verbal behavior accompanying the oral message. The ability to read and decode this leakage is of invaluable aid to the trial lawyer. It can be used in detecting deception during the interview or interrogation; it can be used in orchestrating your conduct and your witness’s conduct during the course of the trial; it can be used to enhance your ability to communicate to the jury or to the court.
Therefore, the reason why body language, or proxemic behavior, is so important is because it entails that the witness will react naturally and automatically to information that may or may not be conveyed with as much certainty as with the use of mere verbiage.