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Before Jim Carrey was attached to the movie, there were a few other actors considered before he was. The first choice for the role of Truman Burbank was Robin Williams, but Williams was only considered for the role. Gary Oldman then started circling the project. He was cast in the role, but Gary Oldman eventually dropped out of the film. After Oldman dropped out, the studio then offered the role to Samuel L. Jackson, but Jackson turned down the role because he wasn't too interested and he didn't fully understand it. Eventually, Jim Carrey was offered the role, to which he said yes.
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.
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.
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.
Philip Glass: Television composer on the synth piano while Truman sleeps and Christof strokes the giant screen.