Bill Paxton was approached to play the diner chef, so he could appear in the second "Predator", "Alien", and "AVP" film in each franchise. However, scheduling conflicts prevented him from making an appearance.
It was, at one time, hoped to include scenes of the Aliens' home world. Conceptual art was created, and it was even storyboarded to be used as the closing shot of the movie, but ultimately, the idea was dropped, in favor of using it in a potential third film.
In the original script, the Predalien was to have died, when the Predator ship crashed, which occurred on page three or four of the script. It was re-written to incorporate the creature into the movie, and make it the main antagonist, as the studio was very impressed by the concept.
According to the DVD commentary, Colonel Stevens (Robert Joy) was originally written as Garber (Adam Baldwin) from Predator 2 (1990). When Baldwin couldn't be scheduled, the character was changed to Colonel Stevens.
The Predator was nicknamed "Wolf" by the filmmakers, after the character Winston Wolfe in Pulp Fiction (1994). His role in the film, like Wolfe's, is described as that of a "cleaner" - one who covers up assassinations, accidents, and other messy situations.
The Predalien was nicknamed "Chet" on-set, and in the script. This was to avoid early spoilers about the nature of the creature (it being a hybrid between the Alien and Predator). The name "Chet" was a reference to the obnoxious brother from Weird Science (1985) (who was played by Bill Paxton).
A voice artist was brought in to perform various Predator noises, after the directors realized that audio samples and tapes containing the original sound effects had been either destroyed, or were of poor quality.
Original music and sound effects were recreated from Aliens (1986). These included the squeals of the hurt Aliens, the sound of the motion tracker, and some of the music, especially at the end. The look of the Alien was also based on Aliens (1986), that being the smooth dome is gone from the tops of the Aliens heads, and is just showing their exoskeletons.
The eight-wheeled armored fighting vehicle, shown in the movie, is supposed to be an M1126 Stryker, derived from the Canadian LAV III, and produced by General Dynamics Land Systems, in use by the United States Army. The vehicle is named for two American servicemen, who posthumously received the Medal of Honor: Private First Class Stuart S. Stryker, who died in World War II, and Specialist Robert F. Stryker, who died in the Vietnam War. The vehicle that is featured in the movie, is actually an Omni International, V-150-S 8x8 APC, dressed up to look like an M1126 Stryker.
(At around forty-three minutes) Jesse stripping down to her underwear at the school swimming pool, is a nod to Ripley stripping out of her uniform in the final confrontation with the Xenomorph in Alien (1979).
The only Alien film in the franchise, that does not feature any eggs that hatch into facehuggers, although a few facehuggers appear in the film, after they escape the crashed scout ship in the opening scene.
The 3rd Alien movie to be given the R16 rating in New Zealand and the 2nd Predator film to be given the R16 rating also in that country. The other films from the Alien and Predator franchises to be given the R16 rating were Alien (1979), Predator 2 (1990) and Alien Resurrection (1997). Later films Predators (2010) and Alien: Covenant (2016) were also given the R16 rating.
The trivia items below may give away important plot points.
AIn the cemetery sequence, in the Unrated Version, the man with a gun stands in front of a tombstone with the name "Hawkins" on it. Hawkins was one of the soldiers in Predator (1987), played by Shane Black, today a Screenwriter (Lethal Weapon (1987)) and Director (Kiss Kiss Bang Bang (2005) and Iron Man 3 (2013)).
According to the DVD commentary, Effects Artists Alec Gillis and Tom Woodruff, Jr. mention that they originally had an effects sequence, that would've shown the Predator skinning Deputy Ray Adams. But the idea was dropped, when 20th Century Fox deemed it "too horrific."
Several sounds are intentionally recycled from previous "Alien" and "Predator" movies as tribute to those films. These include, but are not limited to: actual Predator (like growls and tracking sounds from the helmet) and Alien noises (like hissing and screeching); the beeping of the motion tracker from Aliens (1986), used in this movie during the opening credits, and as the sound made on the tracking screen showing the bomber heading towards its target; and the computers around Colonel Stevens make the same sounds as the Mother computer from Alien (1979). The only original sound that could not be re-used, was the Predator's characteristic chirruping sound, which was recreated specifically for this movie.
This film marks the first, and so far only, time in a Predator film, where the Predator kills a female, when the Predator "Wolf" bisects Jessie with an acid drenched shuriken in the hospital sequence. Although it was an accident, as the shuriken struck two aliens before it pinned her to the wall and sliced her at the waist.
Something that was never seen before in an Alien film, the Predalien is capable of implanting embryos down people's throats using its inner jaw. It was explained by the Strause brothers, that the Predalien was a Queen, that used this method as a temporary mechanism before maturing enough to lay eggs. This was also touched upon before, in the assembly cut of Alien 3 (1992), where a special type of facehugger is seen capable of implanting two embryos, one, a regular Alien, and the other, a Queen, inside Ripley and an Ox (a dog in the theatrical cut), which explains how there could be two Aliens in the film, despite what appears to be only one egg on the Sulaco. This facehugger's corpse can be seen next to the ox's dead body for a brief second, despite the facehugger clearly looking like a normal one in the opening scene of the film. Despite this, AVP: Alien vs. Predator (2004) shows that a normal facehugger impregnated the Predator, which would eventually become a host to a Predalien. This is either a continuity error, or a deliberate retcon by the directors.
Two children, named Kendra (Shareeka Epps) and Curtis (Meshach Peters), were characters that were included in the film during the hospital sequence (as shown by behind the scenes photos), but were cut out of the theatrical release. Curtis was hospitalized with an injured hand, with his sister Kendra keeping him company, the children hear the Predalien in the hallway, and accidentally knock something off a table and draw its attention away from the maternity ward, as they proceed to flee, the Predalien is distracted by the nurse leaving Sue's room, and proceeds to kill her (in both versions, the Predalien merely looks at the maternity ward, before it cuts to the nurse being attacked and killed), the children escape with her car, only for an Alien to jump onto the roof, and attack them before they encounter Dallas and his group on the road. As they fling the Alien off of the car, it proceeds to attack Molly, only for Kelly to kill it with a rail gun mounted on the tank (this shot can be seen in the trailer), the siblings join them, only to leave with Morales, and stay in the town, while everyone else leaves, as they refuse to return to the hospital. Despite this, Curtis' name still appears in the credits.
This film depicts the first time in the Alien franchise, scenes where it appears that an Alien is actually eating, something that was never touched upon in the previous films, as it was implied that they don't need to feed for sustenance. This is shown when an Alien drags Nick's mutilated corpse back into the swimming pool, and proceeds to jab it's inner jaw into him several times, before being killed by the Predator, and when the Predalien forces embryos down pregnant Carrie and Sue's throats, and the Aliens presumably feed on the fetus before bursting out of their stomachs.
The scene where Carrie is cornered by an Alien in the restaurant kitchen, and comes at face level with an Alien, is a reference to the iconic image from Alien 3 (1992), where the Alien is right next to Ripley's face.