Some shots were filmed in downtown Des Moines, IA.

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.

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.

A lot of the scenes involving the meetings with Donald Sutherland and company were actually shot in Fresno, California. To which Keith David was not exactly thrilled to be working there stating "It's a good place to pass through, but"

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," series of movies.

Features cast members from similar-themed genre films: Alien (1979) (Yaphet Kotto), The Thing (1982) (Keith David), and Invasion of the Body Snatchers (1978) (Donald Sutherland).

When the slug has taken over a human, they immediately want to "merge" with one another. The tentacles form the shape of a human hand and interlock themselves with the other slug and stand to transmit information from one host to another and back to the central Hive.

Some shots of the film were filmed in downtown Fresno, California in 1994.

Robert Heinlein's original book took place in 1997, which is also the same year that his other popular novel, "Starship Troopers" was released as a co-production by Sony and Disney under their Touchstone Pictures banner, which produced this film under their Hollywood Pictures banner.

Of all the films in Julie Warner's career, this film is the most popular despite the fact she was in the hit comedies "Doc Hollywood" and "Tommy Boy" with many of the films fans talking to her about it during Sci-Fi conventions when she makes appearances.

At the time of filming, Yaphet Kotto and Richard Belzer were the stars of the hit series "Homicide: Life On The Street"

Donald Sutherland and Eric Thal previously appeared in Six Degrees of Separation (1993).

In the French dubbed version Donald Sutherland's voice is dubbed by actor Jean-Pierre Cassel.

A few members of the cast that are apart of Donald Sutherland's group were featured on the TV series "Law & Order" during its 20 year run or the shows' spin-offs (Criminal Intent, Trial By Jury, Los Angeles and Special Victims Unit) with some playing recurring or different characters. These actors include Tom Mason, Gerry Bamman, Sam Anderson, Andrew Robinson, and even Eric Thal appeared on one of the series at one point. Richard Belzer would join the cast of Special Victims Unit, five years after this film would be completed.

Will Patton and Keith David later appeared in Armageddon (1998).

Donald Sutherland and Marshall Bell later appeared in Virus (1999).

In the film, it is not stated where the alien slugs come from exactly by Julie Warner's or Will Patton's character. However, in the new Special Edition Blu-Ray, Director Stuart Orme stated that they come from one of Jupiter's moons called Titan.

The normal human body temperature is 98.7 degrees. The creature it attaches itself to its' human host tuns them very hot like an engine motor enhancing their hosts strengths and speed.

The whereabouts of the Alien Hive are close to the river in Iowa which are close to cities which is how Sam figured out that Des Moines would be where Mary would be along with everyone who has been taken over to gather together not only do defend but to also completely take over the population within hours.

It takes about five minutes or so for the alien slug to reproduce itself once it's on a human host. This was plainly evident after Mary was taken over and was seducing Sam slowly so that the slug that would eventually attach itself to him would be ready once Mary has him under her control.

In Robert Heinlein's book, the characters eventually found the weakness of the alien parasites/slugs in order to defeat them by crushing them. That was changed for this film and there was three ways to defeat them. The first was by electricity or electroshock to get the creature off its host (ie. Sam in the lab before the creature tries to kill him by stopping his heart completely), The second, was shooting or wounding its host or in some cases shooting the creature itself but it also causing its' host to die along with it. Lastly, a simple sickness or disease that affects the human brain but doesn't kill the person, which was the resolution that David S. Goyer had in his draft of the script.

The idea during the finale when Sam is battling the final alien slug on his father to shock him to get it off him with the shot out helicopter dash was in David S. Goyer's draft of the script.

When a victim of a slug attaching itself to their host is seen throughout the film, they are seen either adjusting the collars on their shirts or or tucking them in for some reason. As seen at the beginning of the film, where Andrew, Sam and Mary are walking through the deserted woods to the bogus UFO ship and people are walking by adjusting their pants or shirts.