Answer: Robin Williams
Robin Williams originally turned down the role because he didn't like the script, but after several re-writes, he agreed to the role. There was also some initial trepidation on behalf of the film-makers that Williams, well known for his improvisation, would not stick to the script, but he agreed to play the role exactly as written.
The director originally wanted Tom Hanks to play the role of Alan Parrish, followed by Bruce Willis, who was unavailable.