As a matter of fact, the entire plot in the original screenplay was completely different than what ended up on screen. The seeds for a crossover between the two franchises was inspired by the 1989 Dark Horse comic series of the same name, which is where the two worlds converged first. Peter Briggs wrote the first version of the script in 1991 with a storyline that closely resembled the comics. 

His script was ultimately rejected, and the film lamented in development hell for almost a decade. Very little of Briggs' version made it into the final screenplay that was re-written by Paul W.S. Anderson (who also directed the film) and Shane Salerno. They drew heavy inspiration from the comic book series as well, but they also included aspects from the AvP video game series and drew on lore from the writings of Swiss author Erich von Däniken, who proposed that extraterrestrial influence explained similarities between ancient cultures. 

Predator 2, which came out in 1990, gave a subtle nod to the comics with an Easter egg. Inside the Predator’s ship, a xenomorph skull was shown amidst a collection of war spoils.

In a 2017 interview with On The List podcast, Sanaa revealed that she initially didn’t audition for the role of environmentalist Alexa “Lex” Woods. The team sent her a copy of the script at the last-minute for her to come in and do a cold reading at an audition the next day. Five days later, she was flown to Prague to do a six-hour screen test, where she landed the role.

“I was dating this guy who I was like so smitten with. He was in L.A. and we were hanging out on a Friday night when I got the call for the audition for like a Saturday. It was a last-minute thing, and I was like, 'Alien vs. Predator... that sounds like a B-movie or something,'” Sanaa explained of her initial hesitation toward the role. “The next day, I was like, ‘I don’t want to go. I don't feel like it. I didn't work on it and I’m not going to get it.’ He was like, ‘Sanaa, you're going,’ and I was like, ‘No, I don't want to go. Let's just go to brunch.'” 

Sanaa was cast as Lex just three days before she had to be on set. Her role took so long to cast because producers reportedly wanted a relative unknown with serious acting chops. She had to leave Los Angeles immediately to begin filming, which took place over the course of six months. 

Sanaa shared that she was in tears on the plane to the shoot location and called the experience at the time probably the most grueling experience of her life because she was not used to being so far away from home. For her, the most exhausting part of working on the film was staying in a constant state of terror.

In The Hunt comic, Machiko Noguchi is the human protagonist-turned-predator protégé who leads Ryushi and forms an alliance with the predators to save the settlement from an alien takeover.

Noguchi was made an honorary Predator and later joins a predator clan at the end of the story. Noguchi would go on to become a series staple and fan-favorite character. In the 2004 film, Lex was also made an honorary predator after helping the predators take down the alien queen. The film version of Lex was also partly inspired by a character from Mark Verheiden's Predators 1991 comic book that closely resembles Sanaa Lathan.

At the end of Alien vs. Predator, one of the predators gifted Lex a spear made by combining a staff with the tail of a xenomorph (a.k.a. alien) after they defeat it in the pyramids. The spear made a brief appearance in the 2018 film right in the beginning.

When biologist Casey Bracket (played by Olivia Munn) arrives at the CIA’s secret bunker for their top secret alien experiment program, Project Stargazer, some trophy alien gear catches her attention. As her gazes over the trophies, it’s hard to miss Lex’s spear from the 2004 film.

As The Hunt, the film stayed in line with the storyline from the comics, which was set sometime in the distant future on the jungle planet Ryushi that had recently been colonized by humans.

Much like the final version, Ryushi is a hunting ground of the predators, and they are returning for their initiation rites when there is an alien outbreak within the human settlement that prompts the predators to team up with the human survivors to stop the aliens from taking over.

Director Paul W.S. Anderson made the call to film the movie overseas because it was cheaper than shooting in Los Angeles. Production began in late 2003 in Prague, where most of the filming took place. The film was set in Antarctica because Anderson felt like it is the most alien-like environment on Earth.

Emmerich turned down the offer, choosing to work on other projects instead. The studio approached Guillermo del Toro to helm the project, who briefly entertained directing the sci-fi flick before opting to make Hellboy instead.

Paul W.S. Anderson ultimately stepped down from directing Resident Evil: Apocalypse (although he did stay on as producer) to write and direct the first Alien vs. Predator film. 

At one part of the film, Lex squares off against an Alien at the base of the ice tunnel.  She says, "You are one ugly mother-," with the last part of her sentence cut of by her firing a gun. Famously, the line was first used by Major Alan "Dutch" Schaefer (played by Arnold Schwarzenegger) in Predator, then by Lieutenant Michael R. "Mike" Harrigan (played by Danny Glover) in Predator 2.

It’s a foregone conclusion that the Alien vs. Predator crossover didn’t live up to expectations of bridging two of the most successful franchises of all time and has been consistently ragged as the worst offering of the expansive series, which tally up to 12 films.

But despite its lack of critical acclaim, the film actually fared better at the box office during its debut than its predecessors and successors. 

It had the biggest opening ever of either franchise at $38 million return over Alien 3's $19.4 million and Predator’s (1987) $12 million premieres.

That also beat out Alien: Covenant's $36 million premiere and topped The Predator's $24 million debut in 2018. It grossed over $80 million at the box office. Ridley Scott’s Prometheus is the only film to have outearned it, opening at $51 million. 

Source: Box Office Mojo

It had twice as many moving parts as the T.rex from Jurassic Park, according to IMDB. Additionally, almost every set in this movie had to be built from scratch by hand.

To bring the film to life, 25 to 30 life-size sets were built, which would’ve cost an estimated $20 million in product costs. In Prague, it only cost $2 million. The pyramid set design was inspired by pyramids built by the Cambodian, Aztec and Jain civilizations.

If, like me, you saw Ridley Scott’s Prometheus or Alien: Covenant, you were probably left feeling a little confused. The two films chronologically threw a few things off as far as continuity goes between the 2004 movie and subsequent films. But the thing is... AvP and its sequel have always stood on shaky ground in regards to their place in the canon of the subsequent films.

Following the film’s 2007 sequel, Alien vs. Predator: Requiem, the two franchises diverged back into their own separate spheres. Prometheus and Covenant completely rewrote the origin story of Weyland Corporation that was previously established in AvP, which essentially nulled its storyline. The Predator made a subtle nod to the 2004 film with Lex’s spear, but the storyline wasn't a factor in the film either.

Weaver was completely opposed to the Alien franchise being made into a cross-over with the Predator franchise. She asked for her character, Ripley, to be killed off. 

“I heard that Fox was gonna do Alien vs. Predator. Which really depressed me, because I was very proud of the movies,” she said in an interview with Entertainment Weekly.  “I heard that the Alien doesn’t beat the Predator and I thought, ‘F— that.'”  

Ridley Scott was also not a big fan of the crossover. Surprisingly, James Cameron, who once said the crossover would kill the “validity” of the Alien franchise, changed his opinion after things were said and done and ranks it as one of his top favorite films from either. 

“Because to me, that was Frankenstein Meets Werewolf. It was Universal just taking their assets and starting to play them off against each other,” he told with film blog Ain’t It Cool in 2006. “Then I saw Alien vs. Predator, and it was actually pretty good. I think of the five Alien films, I'd rate it third.”

The mechanism dripped goo into a bucket in between takes. Sanaa said she got slime in her mouth “several times," according to AMC Story Notes. 

"When I had to scream, the goo got in my mouth. It was gross.”

It's filmed exactly like the shot in Alien (1979) where Ripley (played by Sigourney Weaver) does the same, looking for the alien, and Aliens (1986), when Ripley pulls herself out of the airlock at the end, according to IMDB.