Hands

Fiddling with clothes and hair

The person is nervous or feels uncomfortable.

Touching or covering neck

The person feels insecure. 

Touching, rubbing face and forehead

Stress, due to a problem waiting to be solved. Touching our face helps to keep calm and concentrate on the problem.

Crossing arms

It can have two meanings, depending on the hand grab; When the hands tightly grip the arms, it shows discomfort. The person tries to shield herself.

When the grip is loose, it means that the person is listening, or waiting for something. Though there might be a third reason: the person simply feels cold. 

A piece of advice: don't cross your arms when talking to others. It can give the impression that you are unfriendly.

Hands in pocket

If both hands are fully in the pockets: the person wants to hide something, or wants to be seen relaxed, “cool”. When thumbs are out: posing, showing off. Maybe he is waiting to be photographed.

Again: don't put your hands into your pockets when talking to others. They might think that you hide something.

Rubbing palms

Positive expectation. The guy just have made a good deal. Be careful. At the end of the dinner the waiter comes to your table and rubs his hands; it is not a sign of a good deal but an expectation of a good tip.

Hands clasped together

Frustration. This sends a message of discomfort and shyness. Do it when you are by yourself, but never in company.

Hands in tower shape

Raised position: confident, giving instruction or advice

Lower position: you look interested, you are attentive and ready to respond

Use both. They show that you are thoughtful and wise.

Holding hands behind the back

The person is a superior, in a position of authority and power. Oh, it is good for you, you think, being fearless. However, this is what other people will think about you:  "this guy is overconfident and he thinks he is superior. I don't trust him." 

Nail biting

You are nervous. Maybe only a bad habit, but you do look nervous. Don't do it public. And try to quit privately, too.