The Really Great Thing About Alien The Week It Came Out in 1979 was...(SPOILERS).
..that nobody knew what the "Alien" monster was going to look like.
With "Jaws," the movie poster itself showed us: the monster would be a shark.
With "Psycho," if you read Bosley Crowther's New York Times review(among many others), you would know that the monster was "an old mother who proves adept at creeping up on people with a big knife, drawing considerable blood."
But with Alien, if you read the first reviews on the first weekend(and I did) and saw the movie that weekend(and I did)...you went in really wondering what the heck this monster was going to BE.
And there were surprises there. For awhile, the alien is the crab-octopus-like creature that hugs itself around John Hurt's face.
Then, the alien is about the size of a large rat when it bursts out of Hurt's chest(killing him) and rushes across a table and out of the room(we ALL jumped at the "birth" -- but laughed a little as the little critter ran the hell out of the room.)
Later..when killing next victim Harry Dean Stanton, the alien reveals itself as much bigger now -- human-height-plus -- (what a quick birth-to-growth cycle!) and we are shown its key 1979 "surprise": a set of hydrolic teeth WITHIN its outer teeth, and those hydraulic teeth are powered to fly right out and smash through the victim's forehead. We'd never seen THAT before.
The movie keeps the glimpses of the grown-alien sparing of the movie goes along -- it is seen in full but for less than one second when it kills Tom Skerritt and then is only glimpsed in later killing moments and then...
...finally at the very end with Sigourney Weaver we get to see the creature "up close and personal" and at length ("sucking in" those hydraulic teeth in a moment of repose).
..and then, at the very, very, VERY end, we get to see the Alien as a cross between a rubber dummy and a man in a suit(which it often was) when the creature is tied onto, and then blown out of the rocket booster shaft.
Within two weeks (one?) I believe that Newsweek magazine or Time magazine, or both, had an Alien cover story and a photo of the fully grown creature within but -- on that opening night in 1979, part of the great fun of Alien was to finally SEE what the Alien looked like, and how it functioned, and how it killed.
We were not disappointed.