Director: Ridley Scott
Ridley: Sigourney Weaver (Alien 2/3/4, Ghostbusters)
Dallas: Tom Skeritt (The Dead Zone, Poltergeist 3)
Kane: John Hurt (Monolith, Dead Souls)
Ash: Ian Holm (Time Bandits, Naked Lunch, Frankenstein 1994)
Parker: Yahpet Kotto (Warning Sign, Freddy's Dead: The Final Nightmare)
First off, if you have not seen this film or know nothing about it, stop reading RIGHT NOW AND GO RENT IT! Sorry about that, but I wouldn't be able to write more than "Space, alien, scary" without ruining large amounts of the film.
This was/is the perfect combination of science fiction and horror. You have a spaceship, a robot, and one creepy critter. The entire cast is all-stars, mostly before they were all-stars. When Yaphet Kotto is 7th billed, you know you have some skill. (I am an admitted Yaphet fan, but come on, the next actor listed is the guy in the alien suit!). One could say this launched Ridley Scott's career as a director, as well as most of the actors.
The overall story is about a tow ship towing a bigger refinery ship back to Earth until it hears some type of beacon from an "uncharted moon". (OK, one nit-pick- asteroids aren't big enough to tow back to Earth?. This is the future, but by the look of the ships, I doubt it is the roughly 10,000 years into the future that it would take to mine out the asteroid belt.) The humans are in cryo-sleep, so the computer wakes them up. Turns out there is an alien vehicle on the moon, so they take a shuttle to take a look. Dead meat #1 takes a facehugger to the face. Panic and evacuation.
Back on board, DM#1 finally wakes up, only to have serious indigestion. I have a feeling that mineral ore was not the only thing being tugged along. Maybe about 15 tons of steroids and human growth hormone? Baby grows up so fast.
So big alien is stalking everyone, man-made machines get confused, and everyone gets scared and/or dead.
With a lesser movie I would give much more detail. This is truly one of the classics. I really can't say anything bad about this (besides the whole asteroid belt thing). And I forgot to mention that H. G. Giger designed the beast. (If you don't know Giger's work, go look some of his work online up and feel free to have nightmares tonight.)