A micro expression is a brief, involuntary facial expression shown on the face of humans according to emotions experienced. They usually occur in high-stakes situations, where people have something to lose or gain. Micro expressions occur when a person is consciously trying to conceal the signs of how they are feeling, or when a person does not consciously know how they are feeling. Unlike regular facial expressions, it is very difficult to hide micro expression reactions. Micro expressions express the six universal emotions: disgust, anger, fear, sadness, happiness, and surprise. Nevertheless, in the 1990s, Paul Ekman expanded his list of emotions, including a range of positive and negative emotions, not all of which are encoded, in facial muscles. These emotions are amusement, contempt, embarrassment, anxiety, guilt, pride, relief, contentment, pleasure, and shame. Micro expressions are very brief in duration, lasting only 1/25 to 1/15 of a second .

How micro movements relate to our personality?

Our personality is developed at a very early age. At the same time we develop our behaviour related to expression. A number of studies show that the way we use our micro movements is determent by our personality type.
To be able to read these movements, we use methods that have their origin in FACS. The Facial Action Coding System or FACS is used to identify facial expression. This identifies the muscles that produce the facial expressions. To measure muscle movements the Action Unit (AU) was developed. This system measures the relaxation or contraction of each individual muscle and assigns a unit. More than one muscle can be grouped into an Action Unit or the muscle may be divided into separate action units. The score evaluates duration, intensity and asymmetry. This can be useful in identifying depression or measurement of pain in patients that are unable to express themselves through words.
The Facial Action Coding System training manual, first published in 1978 with multimedia supplements, is designed to teach individuals how to detect and categorise facial movements. The guide provides lessons and practice for memorising action units and combinations of action units. The manual's purpose is to enable practitioners to recognise different physiological attributes of facial expressions, but does not discuss the interpretation of this data. Users should not expect to become face-reading experts! It can be particularly useful to behavioural scientists, CG animators, or computer scientists when they need to know the exact movements that the face can perform, and what muscles produce them. It also has potential to be a valuable tool for psychotherapists, interviewers, and other practitioners who must penetrate deeply into interpersonal communications. A new version (2002) of FACS by Paul Ekman, Wallace V. Friesen, and Joseph C. Hager is now used with several core improvements, including more accurate representations of facial behaviours and cleaner, digital images.