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The spacesuits worn by Tom Skerritt, Sir John Hurt, and Veronica Cartwright were huge, bulky items lined with nylon and with no outlets for breath or condensation. As the actors and actress were working under hot studio lights in conditions in excess of one hundred degrees Fahrenheit (37.8 degrees Celsius), they spent most of their time passing out. A nurse had to be on hand at all times, to keep supplying them with oxygen. It was only after Sir Ridley Scott's and cinematographer Derek Vanlint's children were used in the suits for long shots and they passed out too that some modifications were made to the costumes.
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.