for whatever its worth, some excerpts from wikipedia...
http://en.wikipedia.org/wiki/Alien_(film)
Dan O'Bannon had made a science fiction comedy film with director John Carpenter and concept artist Ron Cobb entitled Dark Star.[14] The film included an alien which had been created using a spray-painted beach ball, and the experience left O'Bannon "really wanting to do an alien that looked real."[14][15] A few years later he began working on a similar story that would focus more on horror: "I knew I wanted to do a scary movie on a spaceship with a small number of astronauts", he later recalled, "Dark Star as a horror movie instead of a comedy."