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."
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.
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.
Philip Glass: Television composer on the synth piano while Truman sleeps and Christof strokes the giant screen.