During the production, MGM opted not to shoot the "nightmare train" sequence to keep costs down. H.R. Giger was not willing to accept that, however, so he spent one hundred thousand dollars of his own money to finance the sequence.
For the scene in which Sil is "born" from the cocoon, the train set was built upside down. Natasha Henstridge, totally naked and covered in KY Jelly lubricant, was pushed through the cocoon, and the shot was flipped upside down afterwards, to achieve the effect of her falling out.
Xavier Fitch (Sir Ben Kingsley) says that the alien signal was received by the Arecibo radio telescope exactly nineteen years after an outbound message was transmitted. Since the transmissions travelled at the speed of light, that means that the alien solar system is approximately nine light-years from Earth. There are two stars which are likely candidates, Sirius and Luyten 726-8A.
Frank Welker, who provided Sil's alien sound effects, said it was one of the most strenuous on his vocal chords with the various types of screaming and screeching sounds he was asked to create, but he had a lot of fun regardless.
When Sil watches television at the motel, she flips through different channels and they all show various things she will do later in the movie. Like having sex, changing her hair, showing off her body, et cetera.
Director Roger Donaldson said in an interview for Starlog Magazine in August of 1995 that the first cut of the movie Editor Conrad Buff put together was two hours and seven minutes long, and it had a different ending action sequence and battle with Sil. A few shots from original ending can be seen in the theatrical trailer.
In scenes of Dan (Forest Whitaker) talking to his therapist, there is a sign on the wall in the background that at first glance is a typical "Thank You For Not Smoking" sign. On closer inspection the text actually reads "Thank You For Smoking".
The premise of Sil's creation, the detection of extraterrestial transmissions that provide detailed instructions for the creation of a humanoid lifeform, is virtually identical to premise of the BBC television series A for Andromeda (1961), co-written by astronomer Fred Hoyle and author John Elliot. The only difference between the two premises is that, in this movie, Earth receives two alien transmissions, the first provides the formula for a new chemical process using methane that will provide virtually unlimited energy (thus convincing the Earth of the aliens' good intentions), then with the instructions for Sil's creation. In A for Andromeda (1961), the Earth receives only one alien transmission, this provided instructions for building a new kind of supercomputer, and the computer in turn generates the intructions for the creation of the humanoid Andromeda.
In August 1995, a woman named Madelyne Tolentino from Canóvanas, Puerto Rico allegedly spotted the chupacabra, which supposedly had killed several animals in the last months. Tolentino later admitted she actually described Sil, the creature from this movie, and believed the events depicted were actually happening in her town.
Dr. Laura Baker (Marg Helgenberger) likens humanity to a galactic weed spreading throughout the galaxy. Other science fiction works that may have inspired it are The Day of the Triffids, Invasion of the Body Snatchers, or Quatermass, which had tripodal Martians.
The motel manager mentions to Sil that she doesn't want her credit card (which she took, along with the clothes, from the train conductor) to fall into wrong hands. Next scene has Fitch telling on the phone that they found her via tracing her credit card.
Two alternate comic book continuations were published - "Species: Human Race" by Dark Horse Comics, featuring a male alien, and the 2005 one-shot special "Species: Offspring" by Avatar Press, featuring Sil herself.
Natasha Henstridge appeared in the short lived comedy Selfie (2014) opposite Karen Gillan. Like Natasha Henstridge, Karen Gillan is a former model turned actress whose breakout role was in the science fiction series Doctor Who (2005), in which he played Amy Pond, companion to the Matt Smith version of The Doctor.
The trivia items below may give away important plot points.
Alfred Molina recalled that Natasha Henstridge was nervous on the day of filming their sex scene since she had never acted in one before. Molina lightened the mood by telling Henstridge that it was a first for him too, since his only previous sex scene was with a man in Prick Up Your Ears (1987).
An early draft of the script had the young Sil calmly killing a friendly cab driver. In an effort to keep the audiences sympathy for her character (and to make the murder of the porter more shocking) the scene was changed to Sil killing a tramp in self defense after he attempts to attack her.
The original, effects-only version of the Sil's nightmare scene where she dreams of herself in her alien form mating with a male alien was twenty minutes long, and was much more detailed and graphic. H.R. Giger, who designed some of the effects for the movie, loved the original uncut version of the scene, and hated that it was only a few seconds long and poorly edited. Giger also came up with the idea of killing Sil by blowing her head off, since he thought that her original death where she gets burned alive with a flamethrower was weak. Giger had some other suggestions for the ending, one of which included a much bigger and more action-packed ending in a drive-in theater showing Ridley Scott's Alien (1979), a movie on which Giger also worked. In that ending, the military fought Sil with helicopters and Laura eventually killed Sil by blowing her head off with a rocket launcher. After she gets killed, Sil's baby crawled out of her, but also died, either by explosion, or by being beaten to death by one of the soldiers.
For Robbie's fatal kiss with Sil, Anthony Guidera and Natasha Henstridge won the 1996 MTV Movie Award for Best Kiss. When receiving the award, Guidera once again kissed Henstridge, then checked the back of his neck and gladly exclaimed "I am alive!" The award was presented to the two by the band KISS, whose bassist Gene Simmons, like Sil, is known for his long tongue.