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Polly Holliday's character of the Hillard family's next door neighbor Gloria Chaney was originally scripted as a large supporting character and had a much bigger role in the film. Many scenes were filmed in which Daniel as Mrs. Doubtfire gets even with Gloria as Gloria tells Mrs. Doubtfire that she heard rumors that Daniel cheated on Miranda and abused his children. This leads Daniel, as Mrs. Doubtfire, observing Gloria's passion for gardening, to give Gloria a formula that will help her flowers bloom better, and the secret ingredient in the formula is dog urine. Several scenes show Gloria actually getting dog urine and applying it to her flowers which kills them, and upsets her very much. Due to time constraints all of these scenes were cut out, and as a result, in the final cut of the film, the character of Gloria is only seen once during the opening credits before she calls Miranda to complain about Daniel throwing a birthday party next door, and later, right before Daniel/Mrs. Doubtfire rips the logo off of Stu's Mercedes Benz, you can see a brief glimpse of Gloria doing some gardening. All of Gloria's deleted scenes can be seen in the special features on the film's DVD.
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).