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.
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.
At the cemetery sequence, 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, 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.
PredAlien's "dreadlocks" were shrewdly composed from the rod puppet alien's tail in Alien³ (1992). This was conceived after running into the logistics of how to convey the PredAlien's traits of being a hybrid Predator/Alien.
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 eight-wheeled armored fighting vehicle is 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 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.
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.
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".
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.