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The early screenplay by Dan O'Bannon and Ronald Shusett contained the Alien, the derelict ship, the Space Jockey, but also an ancient pyramid-like structure on the planet's surface full of alien eggs (more shaped like canisters), where the crew of the Snark (later renamed Nostromo) was to encounter the facehugger. The notion was that the pyramid, eggs, facehuggers and Alien would all be indigenous to the planet, and the Aliens had decimated its original civilization. The derelict ship was a research vessel that had landed much later on the planet, and its pilot, the Space Jockey, had become another unfortunate victim of the facehugger. The pyramid was later re-designed as a honeycombed egg silo because a pyramid wasn't felt to be extraterrestrial enough. However, producers David Giler and Walter Hill initially axed the idea altogether: they didn't like the ancient civilization, and wanted to replace it with a secret Company research installation, complete with weapon testing grounds and an army bunker; the Snark crew would be lured to the planet to be test subjects for the alien eggs, another Company invention. O'Bannon and Shusett were not amused by these alterations, since they had already enlisted Swiss artist H.R. Giger to create an alien landscape and architecture. When director Sir Ridley Scott joined production, he sided with them, and after some back-and-forth discussing, they finally settled on the original idea, but with the egg storage merged with the derelict ship due to time and budget restrictions. Still fascinated with the pyramid idea, Scott hoped to use it in a sequel one day that could explore the origin of the creatures. The pyramid-like structure filled with eggs/canisters would later resurface in Alien vs. Predator (2004), and, as predicted, in Scott's origin story Prometheus (2012).
The blue laser lights that were used in the alien ship's egg chamber were borrowed from The Who. The band was testing out the lasers for their stage show in the soundstage next door.
The chestbursting scene was NOT filmed in one take (despite the myth). The scene was filmed twice: on the first take, the chestburster was unable to cut through Kane's shirt, so the crew needed to reset and shoot it again. The failed attempt is visible in the finished film, since director Ridley Scott thought it made it look like the creature was struggling to push its way out, and made the scene more violent (see The Beast Within documentary where this is discussed). According to designer Ron Cobb, the two takes of the scene were shot from multiple angles, and about 40 minutes of footage was available for screening.
Shredded condoms were used to create tendons of the beast's ferocious jaws.
According to Yaphet Kotto, Sir Ridley Scott told him to annoy Sigourney Weaver off-camera, so that there would be genuine tension between their characters. Kotto regretted this, because he really liked Weaver.
To get Jones the cat to react fearfully to the descending Alien, a German Shepherd was placed in front of him with a screen between the two, so the cat wouldn't see it at first. The screen was then suddenly removed to make Jones stop advancing and start hissing.