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The rumour that the cast, except for Sir John Hurt, did not know what would happen during the chestburster scene is partially true. Everyone had read the script, which explicitly stated that something would be coming out of Kane's chest, but they did not know specific details. For instance, Veronica Cartwright did not expect to be sprayed with blood, so her reaction to it was genuine. Sigourney Weaver related that they suspected that something dramatic was about to happen, because when they got to the set, transparent screens were set up and the crew were wearing raincoats. Tom Skerritt admitted years later that he was the only one besides Hurt to know exactly what was coming. He had been following director Sir Ridley Scott around to learn about the process of filmmaking, and had been present during meetings where the chestburster effect was discussed in detail. He was of course requested to keep the specifics to himself, in order to elicit genuine reactions from the other cast members.
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 did not make it through Kane's shirt, so the crew needed to reset and shoot it again. The failed attempt is visible in the finished film since Ridley 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).
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.