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A scene scripted, but only partially filmed, was referred to as the "Air Lock scene". It was supposed to take place after the scene where Ripley finds out about Ash's special order. Parker contacts Ripley and Lambert over the intercom, saying that he is watching the Alien moving in a corridor near an inner airlock door. He asks Lambert to open the inner door from the bridge, in the hope that the Alien will enter the air lock, so they can close the door and blow it out into space. Ripley makes her way to Parker's position, and the plan seems to work, until an alarm suddenly goes off. It startles the Alien, and as it escapes the air lock, its tail gets crushed under the inner door. Its blood causes a small hull breach, which temporarily decompresses the section, knocking Ripley and Parker unconscious. After being revived, Ripley is convinced that Ash set off the alarm in order to protect the Alien. This leads to the scene where Ash traps and assaults Ripley (her nosebleed was meant to be an aftereffect of the decompression). Only the shots from the Nostromo's bridge were filmed, but the remainder of the scene had to be scrapped, due to time limitations.
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.