Jennifer Jason Leigh insisted on keeping one of the prop pods with her in between filming to strengthen the maternal bond you see in the film.
Jennifer Jason Leigh had already finished shooting her role in Eyes Wide Shut (1999) when she took on this role. When her scenes in that movie required re-shooting, the schedule required for it interfered with this one. Leigh chose to stay on this movie, and her role in Eyes Wide Shut (1999) was re-cast.
At one point during the movie's development, it was with American studio Metro-Goldwyn-Mayer. One reason that director David Cronenberg eventually left the studio and took the project elsewhere was because he wanted to cast Jennifer Jason Leigh in the movie, but a MGM executive said that he associated Leigh with "little art movies that don't make money."
Two producers of this movie are Hungarians, so it is not by chance that the X and the Z of the word "eXistenZ" are capitalized, since the letters between them make the Hungarian word "isten", which means "god".