The 1979 movie Alien broke new ground in science fiction and horror. The slow reveal of the xenomorph and the slow reveal of who the real hero of the story was combined to make a darn good story -if you saw it in a theater. But if you saw it much later, after both became well-known, it was still a thrill. The production was not simple at all. There was a lot of technical stuff that didn't go as smoothly as planned, and weird things going on off-camera.
Read lots more trivia about the production of Alien at TVOM.