He was portrayed by Martin Sheen in the movie, and by Sean Patrick Flanery in the series.
Greg Stillson is a mentally unstable politician who wants to be elected President of the United States. During a rally in the town of Trimbull, he shake hands with the psychic Johnny Smith, who has a vision of Stillson becoming President and starting a nuclear war. A few days later, Johnny decides to assassinate Stillson and tries to shoot him during another rally. Unfortunately, he misses several times and Stillson grabs a child in order to use him as a human shield. Johnny is then shot and mortally wounded, but before dying he has a vision of Stillson losing elections.
In the film, Stillson commits suicide after losing the elections.