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Talk of a sequel began in 2003, with a script being written by Bonnie Hunt. Robin Williams was set to return in disguise as an old nanny like in the first movie. Due to problems with the script, re-writing began in early 2006 as Williams was allegedly unhappy with the plot. The film was expected to be released in late 2007, but following further script problems, the sequel was declared "scrapped" in mid 2006. The sequel's story was originally said to involve Williams, as Mrs. Doubtfire, moving close to his daughter's college, so he could keep an eye on her. Serious discussions regarding the sequel re-ignited in April 2014, with an announcement that Williams and Chris Columbus would be teaming up with Fox 2000 Pictures to produce the sequel. Williams' sudden death just four months later ultimately sealed the project's fate once and for all. No one replaced him either.
According to one biography, Robin Williams decided to test out the believability of his Mrs. Doubtfire character during filming by going as Mrs. Doubtfire into an adult bookstore and making a purchase. He was able to do so without being recognized.
During the scene when Mrs. Sellner comes to inspect Daniel's apartment and Daniel/Mrs. Doubtfire is serving her tea, the icing on his/her face is melting off. This was not intentional. The heat from the set lights melted the icing on his face and Robin Williams improvised the bulk of that scene.
Chris Columbus would use two or three cameras at a time when shooting Robin Williams' scenes, uncertain of what the famously improvisational actor would come up with. Columbus viewed shooting these scenes as if he were making a documentary.
Chris Columbus was amazed how far Robin Williams took his performance. First, he played each scene as scripted two to three times, and then was allowed to improvise, or "playing" as Williams called it. Columbus allowed Williams a lot of improvisation, because that was where the film's funniest material came from; in fact, Columbus called it magical at times.
According to director Chris Columbus, Robin Williams improvised so much that there were PG, PG-13, and R edits of the film (though always intended to be released as PG-13).