Mon Oct 5
2009  
Alien uses a tricky device to keep the alien fresh throughout the movie: It evolves the nature and appearance of the creature, so we never know quite what it looks like or what it can do. We assume at first the eggs will produce a humanoid, because that’s the form of the petrified pilot on the long-lost alien ship. But of course we don’t even know if the pilot is of the same race as his cargo of leathery eggs. Maybe he also considers them as a weapon. The first time we get a good look at the alien, as it bursts from the chest of poor Kane (John Hurt). It is unmistakably phallic in shape, and the critic Tim Dirks mentions its “open, dripping vaginal mouth.”

Yes, but later, as we glimpse it during a series of attacks, it no longer assumes this shape at all, but looks octopod, reptilian or arachnoid. And then it uncorks another secret; the fluid dripping from its body is a “universal solvent,” and there is a sequence both frightening and delightful as it eats its way through one deck of the ship after another.

[…] the alien is capable of being just about any monster the story requires. Because it doesn’t play by any rules of appearance or behavior, it becomes an amorphous menace, haunting the ship with the specter of shape-shifting evil.
Roger Ebert on “Alien”