Dark Horse recently released an adaptation of Alien: The Original Screenplay, but what many don't know is that there were some stark differences between the original script for the Ridley Scott movie and what fans eventually saw play out on the big screen.
Dan O'Bannon started working on this script after working alongside John Carpenter on the movie Dark Star. The original script by O'Bannon had the title Starbeast, later changed to Alien when the final product arrived. The shooting script included significant uncredited re-writes by Walter Hill and David Giler, making it a very different beast.
When looking at the original script's characters, none of them remained for the actual Alien movie. They included a captain named Chaz, his intelligent executive officer Martin, the brash glory-hound navigator Dell, the intellectual romantic communications tech Sandy, the high-strung mining engineer Cleave and the hard-working engine tech Jay.
This crew was not the down-and-dirty scavengers that set out to encounter the aliens in the movie. The entire crew's archetypes changed for the better in the final version of the film. O'Bannon wrote into the script that all the characters were unisex, and changes to them would not alter his story. However, the most significant change ended up altering the movie for the better.
The difference, and what changed Alien into something special when it hit theaters, was Sigourney Weaver as Ripley. The O'Bannon original script had no character named Ripley and no female characters in general. The end of the basic synopsis of the film even included the notation that there was only "one man left alive" after finally eliminating the creature. The crew in the original script was male, and that changed the dynamic that helped the franchise rise about the rest of its genre. It was the strong female character that went toe-to-toe with the Xenomorph that made the movie so memorable. Without Ripley, Alien wouldn't have stood the test of time as the iconic film it is.
The original script also saw the Nostromo called Snark, which was another change for the better. The next big difference was the reason that the Snark made its trip to the alien craft. In O'Bannon's script, it was all about the idea of finding extraterrestrial life, what the crew called making "first contact" with aliens. However, the movie changed it to a more reluctant landing, forced to rescue a downed ship, a mission the crew's contracts required. The one big difference here was that the crew in O'Bannon's script voluntarily went to the alien craft because they believed they could be the first to make alien contact.
In the movie, they had no choice in their eventual fate. This fact leads to another significant change from the script to the film. In O'Bannon's original script, Ash did not exist, and there was no android member of the crew. The android was something Hill and Shusett added in their version of the script, and much like Ripley, became a massive part of the franchise as the movies came out. The fact that a character betrays the crew to try to capture the alien is not in the main O'Bannon script. Ash added a secondary plot to the story, making it more than a straightforward monster movie. With Ash taking the crew to their deaths, the real villain was an android and not necessarily the alien.
There were also visual changes, as well as a tightening of the script. Notably, the introduction of the Face Hugger was changed. In the movie, Kane finds a room full of giant eggs on the floor, with a mist rising around them. It is from one of these eggs that the Face Hugger leaps out and attaches it to Kane's face. In the script, it was less impressive, described as a pedestal covered with what looks like rows of leathery jars. Seeing the room full of giant eggs made a visual impression that a pedestal of jars could never have accomplished.
There were also minor differences most likely changed for pacing issues as well as the length of the movie. In the original idea, there was a lot of discussion about pyramid designs and hieroglyphics found in the alien ship. These came into play later in the franchise, but here they ended up cut from the final product. On top of that, the dead Face Hugger that Ash demanded they keep to study was thrown out of the ship and onto the planet in the original script before the crew took off.
Finally, the battle with the Xenomorph was different. In the original script, it was more of a team effort as they tried to hunt down the alien, losing every step of the way. However, in the movie, it played out like a slasher flick, with the alien taking out each crew member one by one (outside of Ash) until it was down to Ripley alone to battle the alien. There was one moment from the original script that Scott shot but then cut in the editing stage of Ripley burning of the two bodies in the cocoons.
After that, things played out with the same basic beats, except for one last jump scare removed where the alien grabbed the sole survivor's legs before getting launched into space. To add intrigue, there was also a stinger at the end in O'Bannon's script where a spore pod was attached to the bottom of the escape craft, showing the battle was not finished yet.