Like Aliens before it, Alien3 was primarily shot at Pinewood Studios, near London, starting in January 14, 1991, without even a finished script and having spent $7 million.[5] The production was infamously fraught, with Fincher experiencing extensive interference from the studio and frequently clashing with 20th Century Fox over where the film should be going. Several actors on set observed that Fox appeared to have hired Fincher on the assumption that he was "a pop video promo director that they could push around", and when this ultimately proved not to be the case, the atmosphere became increasingly hostile.[15] To add to these problems, Fincher had to deal with a script that continued to change even as filming was taking place; according to the director, rewritten pages would arrive at the studio via fax machine to be filmed the following day, a scenario he labelled "just insane".[16] The story also had to be tweaked to allow expensive sets that had been constructed for earlier iterations of the story to be used in filming.[1]
Eventually, overruns at Pinewood caused filming to be shut down, and the production moved to Fox Studios in Los Angeles, California.[1] Reshoots of the ending involving Sigourney Weaver required the construction of an incredibly expensive fake bald cap, as the actress refused to shave her hair again.[1] So chaotic was the final post-production process that the structure of the film was being significantly altered even at a very late date. Most notably, a sequence in which the prisoners attempt to trap the titular Alien inside a waste tank was originally intended to succeed, but this was altered shortly before release in response to test screenings in which some viewers expressed the opinion that it made the Alien appear weak; the change was made so late that the original version appears in almost all of the film's related media, including the novelization, the comic adaptation and even the film's trading cards. In fact, the film's ending scene was still being edited just three weeks before the movie was due to be released in the United States.[17]