Russian President Putin is not calm all the time. He has a decent range of emotional displays. One reason is culture. People raised in individualistic cultures have a wider range of emotional display than people in collectivistic cultures.
Putin is an ethnic Russian and was raised in a more collectivistic culture (the USSR) than many people who perceive him to be calm all the time.
Another reason is that Putin is a former KGB operative (and a skilled one). These people learn how to stay calm in stressful situations as part of their basic training. Here is the quote from recently deceased U.S. Senator John McCain: "I looked into Mr. Putin’s eyes, and I saw three things: a "K" and a "G" and a "B."
Yet another reason for it that Putin is a leader of a great country and he is expected by his inferiors to maintain calm and focus in all kinds of situations. In other words, he has to look and act like a Tsar(Emperor). He is not tall, but he needs to look formidable. So, his posture is firm and steady, he tends to look directly into people’s eyes, a bit confrontational, but in a manner expected of a Tsar.
Putin has a dominant personality. He enjoys power over others and expects obedience and respect. Finally, Putin's face has been smoothed by generous infusions of Botox. Botox relaxes your facial muscles and make your face appear static.