Bill Paxton was approached to play the diner chef so he could appear in the second 'Predator', 'Alien' and 'AvP' film in each series. 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 Pred-Alien was to have died when the Predator ship crashed which occurred on page 3 or 4 of the script. It was re-written to incorporate the creature into the movie and make it the main villain as the studio was very impressed by the concept.
According to the DVD commentary, Robert Joy's character Col. Stevens was originally written as Garber from Predator 2 (1990), who was the highest ranking survivor from Peter Keyes team hunting the predators. When the actor who played Garber (Adam Baldwin) couldn't be scheduled, the character was changed to Col 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 (i.e. it being a hybrid between the Alien and Predator). The name "Chet" was a reference to the obnoxious brother from the movie 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 of poor quality.
Original music and sound effects were recreated from Aliens. 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 from Aliens, that being the smooth dome is gone from the tops of the Aliens heads and is just showning exoskeleton.
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: Pfc Stuart S. Stryker, who died in World War II and Spc4 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 a M1126 Stryker.
(at around 43 mins) 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 the original Alien (1979).
The only Alien film in the Alien franchise that does not feature any eggs that hatch into facehuggers, although a few facehuggers themselves appear in the film after they escape the crashed scout ship in the opening scene.
At the cemetery sequence (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 of Dutch (Arnold Schwarzenegger) on the first Predator (1987) movie, played by Shane Black, today a screenwriter (Lethal Weapon franchise) and director (Kiss Kiss Bang Bang (2005)).
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 us the Predator actually skinning the body of Deputy Ray Adams. But the idea was dropped when Fox Studios that is was deemed "too horrific."
The Yutani character (of the "Weyland-Yutani" Company from the "Alien" films) was originally going to appear in the AVP: Alien vs. Predator (2004), but was written out. The character was originally conceived to be male.
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 Col. Stevens make the same sounds as the Mother computer from Alien (1979). The only original sound that could not be reused was the Predator's characteristic chirruping sound, which was recreated specifically for this movie.
Steven Pasquale's character "Dallas Howard" shares his name with Tom Skerritt's character from the original Alien (1979) movie - Dallas was the captain of the Nostromo. Moreover, he says the line "Get to the chopper!", which was also said by Dutch (Arnold Schwarzenegger) in Predator (1987). Perhaps a coincidence, he also shares his name with actress Bryce Dallas Howard who does not appear in any of these series.
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 inferred 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 in the school 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 stomach.
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 of the film. 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 of babies, as they proceeded to flee the Predalien is distracted by the nurse leaving pregnant 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 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 refused to return to the hospital. Despite this Curtis's name still appears in the credits.
This film marks the first, and so far the only, time in a Predator film where the Predator creature kills a female character, 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.
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 Alien³ (1992) where the Alien is right next to Ripley's face.
Something that was never seen before in an Alien film, the Predalien is capable of implanting embryos down people's, or as seen in the film, pregnant women's throats using it's 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 Alien³ (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, 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.