Shared with you
The sets of the Nostromo's three decks were each created almost entirely in one piece, with each deck occupying a separate stage and the various rooms connected via corridors. To move around the sets, the actors and actresses had to navigate through the hallways of the ship, adding to the movie's sense of claustrophobia and realism. The sets used large transistors and low-resolution computer screens, to give the ship a "used", industrial look, and make it appear as though it was constructed of "retrofitted old technology". This added to the visual idea of a "used future", where future technology is presented as lived-in, having been around and used for years, rather than being pristine and brand-new. Upon the release of the prequel Prometheus (2012), Sir Ridley Scott added that the Nostromo was a particular case of a worn-down, decades-old towing vehicle full of obsolete and low-tech machinery.
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.