Back in 1979, Alien was not the horror sci-fi classic it is today. In fact, 20th Century Fox was totally unsure of what up-and-coming director Ridley Scott had made. You must remember that he was pretty much an advertisement director up until that point with one feature to his name, the 19th-Century period piece, The Duellists (1977).
When an early test screening convened in Dallas, Texas (the second one after the sound didn't work at the initial screening in St. Louis), Scott was so nervous, that he couldn't even stay in the theater. More than that, he had to get a little sloshed.
"I kept having to talk around the block," he told Empire Magazine for its definitive history of the Alien franchise, adding that he would grab an alcoholic beverage at a local watering hole. He'd return to the theater and ask, "Where are they now?" before leaving again.
When he came back with half an hour of the movie left, he found the theater in complete disarray: the theater manager was white as a sheet, the women's bathroom was coated in puke, an usher had fainted, and one enterprising individual had "even stuffed a towel against the speaker to shut out the sound piped in from the auditorium." It's a story Scott has talked about before, but it was the moment that all of his fears melted away; the reaction of the audience and theater staff confirmed that he'd made a legitimately scary film.
One thing that made the movie so effective was the sense of claustrophobia, the idea of the Nostromo's crew members stuck in a tiny place with the ultimate killing machine. Indeed, Scott progressively closed in the walls of the labyrinthine set as production went on to underscore that sense of being utterly trapped.
The late John Hurt (Kane, the first-ever victim of the chestburster) once remarked that "there was no escape."