Joseph Stalin was one of the most important figures in the entire Cold War. You could say that he was the cause of the Cold War. Whether he was the cause or not, he was certainly one of the most important actors in the beginning of the Cold War.
Stalin was the leader of the Soviet Union from the 1920s until his death in 1953. This means that he was the center of power in the USSR during the years in which the Cold War was beginning. His attitudes towards the West and towards Eastern Europe helped to bring about the Cold War.
Stalin wanted to have a large buffer zone of Eastern European countries to be under his control. He wanted them to protect the Soviet Union from invasion from the West. Because of this, he was determined to keep control of Eastern Europe after WWII. His determination to dominate Eastern Europe helped to scare the West and push it to take a more aggressive anti-communist line. Stalin’s actions, such as the blockade of West Berlin, helped to solidify the antagonism between the two sides.
Thus, what Stalin did in the Cold War was to help create the atmosphere of mutual animosity that drove that conflict.
I believe one of the most significant thing that Stalin did (as said above) was the remove democratic ideas. To enforce this, he also created the Secret Police, if you said anything or have done anything against the Government, you would be kidnapped and eventually killed. That also includes people to run for the Government against Stalin, he will eventually take out all enemies that are running against him.