For the seduction scene where Mary tries turn Sam back with one of her alien slugs, Julie Warner had to wear an apparatus that looked similar to her back with the creature's movements guided by a team of puppeteers and make up effects crew.
No less than nine writers worked on the script. Besides the credited writers Ted Elliott, Terry Rossio and David S. Goyer, work with also done by James Bonny, Richard Finney, Michael Engelberg, Neal Purvis, Robert Wade and the film's director, 'Stuart Orme. The final version mainly uses ideas from the Goyer and Orme rewrites.
The hive or the Central Intellegence home where everyone gathers to collect intelligence from their human hosts was actually a set built on the Disney lot. While the actual entrance of the nest was at the Los Angeles County Center, with styrofoam walls built there to make it look like it was taken over.
At the start of the film when the boys are trying to convince Mary to go inside the fake spaceship, Andrew Nivens says, "She balked at Pirates of the Caribbean." Both Ted Elliott and Terry Rossio, two of the films credited screenwriters, went on to write the "Pirates of the Caribbean" films.
Some shots were filmed in downtown Des Moines, IA.