Introduction
Laymen explanation
Technical explanation
Body language is a type of a nonverbal communication in which physical behaviors, as opposed to words, are used to express or convey the information. Such behavior includes facial expressions, body posture, gestures, eye movement, touch and the use of space. Body language exists in both animals and humans, but this article focuses on interpretations of human body language. It is also known as kinesics.
Tips
Look at person face while talking. It will give indication about his/her reaction on what you are communicating to him/her
Emotions can also be detected through body postures. Research has shown that body postures are more accurately recognised when an emotion is compared with a different or neutral emotion
Physical expressions
Happiness
Sadness
Focussed
when a person is focused their eyebrows are lowered and more centred.
Unfocused
An unfocused facial expression will often feature the eyebrows being raised with an unfocused look to the eyes.
Confident
Confident facial body language involves a more focused, centred and energised look
Afraid
Leg shaking while listening
The faster the shaking, the stronger the disagreement. Refer here
Verify it
A lack of wrinkles around the eyes suggests a potentially fake smile. At one point, researchers believed that making a genuine smile was nearly impossible to do on command. When someone is smiling joyfully they wrinkle around their eyes. When someone is faking it, they do not. If someone is trying to look happy but really is not, one will not see the wrinkles.
The person has no control over his pupils and they expanded when someone was interested in another person, or when they were looking at something.[14] “As an indicator, check a friend's pupil size when you're talking to them about something interesting, then change the subject to something less interesting and watch their pupils contract!”.[14]
Normally, one's eyes need to instinctively blink at around 6-10 times per minute, but merely looking at a person or object the viewer finds "attractive" can slow this rate down and can be a good indicator that a person is attracted to the person they're talking to, and, thus, it may be a sign of flirting.
Generally, deeper breathing which uses the diaphragm and abdomen more is interpreted as conveying a relaxed and confident impression; by contrast, shallow, excessively rapid breathing is often interpreted as conveying a more nervous or anxious impression
Reference
https://en.wikipedia.org/wiki/Body_language