Depending on the mood or look I am going for, I try to give the subject something to think about so that they are not too focused on the work of posing. When some people pose they lock up and are not always natural looking. I create a scenario for them to imagine themselves experiencing so they will not be as nervous and try to express the look that we are wanting to achieve. Directing the model is just one of the factors a photographer has to control during a shoot. Along with lighting, setting, and environment, there is an element of timing that is involved. The most extreme scenario I once used for a night beach photo shoot was for the girl to imagine that she was on a boat with someone but got thrown off of the boat and had to swim ashore for survival so she is disillusioned, upset, and tired. Originally she was a bit stiff, nervous, and not really pulling off the look. But then she was able to laugh at the thought of the scenario and it worked out.

Source: How are the professional portrait photographers capturing the perfect moment? – Quora