Alien is a terrifying movie, that relies mostly on great special effects and a smart plot. Scott magistrally develops the haunted-house story about seven astronauts incarcerated in a spaceship with a perfectly lethal being. Not much about the personality of the characters is told in the beginning. We know that they're running into a dangerous, unknown place, from which they might not get out alive, but their emotions are unbearably controlled and restrained. As the menace of the Alien becomes clearer, we start to make a better idea about the character's actions and thoughts.
The Alien itself is slowly introduced and not fully shown throughot the entire movie. It reminded me of Jaws. The effect it achieves is very similar too. The fear of the unknown, of a monster who is near but still far from everyone's eyesights, is wonderfully conducted. The creature is terrifying, but its purposes (he probably has none, as long as no regrets) and killing methods are never clear to us, as they aren't clear to the spaceship's crew.
One of Scott's best insights is not to let the audience become too involved with the characters (except for Ripley, a courageous, strong woman, that smartly becomes the only survival of the struggle). In a very strange (but still crucial) scene, Harry Dean Stanton's character is said to be someone with no personality, who only agrees with what his partner tells him. It is, as we will become aware, a hint on the next sequences. He is the perfect first victim for the bloody creature, walking carelessly into certain death. The great achievement of Ridley Scott's direction is the feeling that we are part of the crew, that we belong to that atmosphere and are in Alien's unmerciful hands.
The art direction and special effects are great, still bright and beautiful for a modern audience's trained look. A masterpiece, as long as its sequel, Alien is a horror movie that contradicts some cliches of the genre and that presents us a great character: the Alien itself, that made its way through the popular imaginarium right after its debut.