During the famous chestburster scene, the actors weren't told how gushingly violent the alien's birth would be (which caused a stream of blood to hit an unprepared Veronica Cartwright (Lambert) square in the face). The result was honest-to-God fright. When the call to "Action!" came, the crew said they didn't have to do much "acting" after that. As Yaphet Kotto (Parker) recalls, "We were freaked. The actors were all frightened. And Veronica nutted out."
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Did you know Ridley Scott has a blink-and-you'll-miss-it cameo in Alien? He does, and here's how to spot him. When Kane shines a light in the alien egg, we can barely make out a facehugger making these squishy, spasmy motions. That facehugger is Sir Scott putting on one heck of a performance. More specifically, it's his hands putting on the performance, costumed in a pair of rubber gloves. The egg for this scene was made of clear fiberglass to give it that eerie, unearthly quality.
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Audiences saw so little of the alien in Alien that is difficult to tell much about it, especially how Ridley Scott and his team pulled off creating such an impressive creature in a pre-CGI days of prosthetics and string. As it turns out, for several scenes, the alien was actually played by Bolaji Badejo. At 6'10, the foreign exchange student from Nigeria towered in the perfect way and his frame made the Slender Man look like a starting lineman.
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Producers David Giler and Walter Hill rewrote Alien several times, and some of their scripts took the story in weird directions. According to Ron Shusett, one of these directions included having the alien do battle with Genghis Kahn, Attila the Hun, Jack the Ripper, and the rest of history's dirty dozen. It sounds crazy, but you have to admit: You do kinda want to see that movie, don’t you?
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The tendons of the alien's jaws were created using shredded condoms. When you consider that the alien looks a lot like a giant phallic symbol and its acts of killing seem a lot like cross-species rape, this is a horrifyingly fitting choice.
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Alien's tagline, "In Space No One Can Hear You Scream," isn't just a perfect encapsulation the film's blending of horror and science fiction. It is also a pretty clever play on the fact that there are no air molecules in the vacuum of space, meaning sound has no substance to travel through. Light still travels through space though, so they’ll at least see the alien noshing on your brains.
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The shot of the astronauts in front of the alien spacecraft was actually done to scale. The actors are children, in order to sell the illusion that the ship was bigger than it actually was. (Source)