The Trumania bit, where Truman draws on the mirror with soap and acts strange, was completely improvised by Jim Carrey. In another take, he drew long curly hair and a dress.
The original script had Truman Burbank living in a recreation of New York City. But Peter Weir changed it to an idyllic town to make the movie feel less "sci-fi."
Peter Weir wrote a 10-page backstory that described the history of the show. For instance, The Truman Show was a frequent winner at the Emmys.
Truman was supposed to be just out of high school, but since Jim Carrey was in his thirties, it got swapped from teenaged angst to more of a midlife crisis.
Philip Glass: Television composer on the synth piano while Truman sleeps and Christof strokes the giant screen.