According to Wil Wheaton, in the scenes that he was in with Robin Williams, they would film a take the way it was supposed to be filmed. After that take, Williams would often want to improvise scenes differently than the script, just for fun. Those scenes were not added to the actual film, but there were enough scenes to make an entirely different movie.
John Hughes' first choice for the role of Professor Philip Brainard was always Robin Williams. He lined up some other choices including Christopher Lloyd, Jeff Goldblum, John Lithgow, Tim Allen, Jerry Seinfeld, John Goodman and Sir Patrick Stewart. As soon as Williams made peace with The Walt Disney Company, Hughes immediately cast Williams in the lead.
Christopher McDonald almost declined the role of Wilson Croft when he was offered it before Robin Williams signed on to star. He felt that the role was too similar to those of the ones he played in Thelma & Louise (1991) and Happy Gilmore (1996). At the time Williams had a falling out with The Walt Disney Company and other actors were considered for the role of Professor Philip Brainard. Williams reconciled with Disney before production and McDonald ultimately accepted the role solely for the reason that he had always wanted to work with Williams and was willing to play any role possible for that opportunity.
In the film poster where Brainard is wearing his goggles and Flubber is not wearing his glasses, a few bubbles of flubber in the upper right-hand corner form a hidden Mickey.
Nancy Olson: a secretary at the Ford Company (uncredited). Olson played the romantic lead in the original film 'The Absent Minded Professor.'