Today, movies from the pre-CGI era can seem cheesy or fake, especially when sci-fi films tried to depict realities no one had ever seen before. Sharp-eyed audiences have spied many a monster's string and complained about the herky-jerky movements of stop-motion animation.
Alien's production screams classic Hollywood. The film was shot on good ol' fashioned film, and principal photography took place on set at Pinewood Studios. During filming, actors walked the halls and air ducts of the Nostromo with nary a green screen in sight. Outer shots of the Nostromo are of a model of the ship, whose building was supervised by Nick Allder and Brain Johnson and filmed at Bray Studios (source).
Another example of this classic approach to film-making is the alien itself. In the pre-motion capture days of 1979, Scott and his crew had to both create a monster costume and find someone to wear it. For the wearing, they found Bolaji Badejo; for the creation, they found H. R. Giger. As he recalls:
So I started with a kind of statue of Bolaji, and directly over that I modeled the shape of the Alien in plasticine, with bones and tubes and lots of mechanical things. The head I built up from a real human skull using plasticine and flexible piping […] See the muscles and tendons of the jaw? We made them out of stretched and shredded latex contraceptives. (source)
We told you there was something phallic about that mouth.
Scott has said of the film design: "I always believe that if you can do it physically do it. You can spend one hundred thousand dollars […] and it's ridiculous. You don't need to" (source). And when it is done right—i.e. you can't see the strings—you get Hollywood magic.