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Several planned but un-filmed scenes were; Dallas and Parker using a craft called "The Flying Bedstead" to enact repairs on the exterior of the ship while in space. A sex scene between Ripley and Dallas in the shuttle before they go to search for the chestburster. The crew using internal cameras to look for the Alien, where they find it halfway matured, looking something like a cross between the chestburster and an egg with feet. Dallas' death was to take place in a huge upside down "wind tunnel" in the air duct system: he looks up to see the Alien on the ceiling of this massive cylinder, where it leaps from one side to the other in a super-fast descent toward him. In the climax, the Alien was to pull Ripley out of the shuttle with the grapple wire where she shoots it with a pistol and makes her way back inside before destroying it with the engines.
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.