If the person you’re speaking to seems unnervingly still, it could be wise to question why — they might be holding back the truth, and showing it through their (lack of) body language. When you speak and engage in normal conversation, it is natural to move your body around in subtle, relaxed, and, for the most part, unconscious movements,” says behavioral analyst Lillian Glass to Business Insider. When someone you’re talking to isn’t doing this, though, “this may be a sign of the primitive neurological ‘fight’ rather than the ‘flight’ response, as the body positions and readies itself for possible confrontation,” according to Glass.
One of the symptoms of the fight or flight response — which is caused by a rush of hormones released in reaction to a perceived threat — is muscle tension, as the body primes itself to fight (per Verywell Mind). When someone’s lying, then, they may unconsciously lock up in preparation for conflict. Therefore, as Glass tells Business Insider, “if you observe a rigid, catatonic stance devoid of movement, it is often a huge warning sign that something is off.”