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Director Sir Ridley Scott originally wanted to use animatronics (remote-operated puppets) to portray the Alien, thereby hoping to avoid the sight of a monster being obviously played by a man in a suit. Unfortunately, the special effects techniques at the time weren't sophisticated enough for what he had in mind, but he agreed to an actor playing the part after being introduced to Bolaji Badejo. Badejo was a graphic artist who was discovered at a pub by one of the casting directors. He was 208 cm (six feet almost 10 inches) tall with very thin arms, just what they needed to make the Alien look less human and more insect-like. He was sent for Tai Chi and mime classes to learn how to slow down his movements. A special swing had to be constructed for him to sit down during filming, as he could not sit down on a regular chair once he was suited up, because of the Alien's tail.
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.