Nominated for three Academy Awards and director of almost 30 movies in as many years, Ridley Scott—sorry, Sir Ridley Scott; he was knighted by Queen Elizabeth II, you know—needs no introduction, but we'll do it anyway. His films have featured a variety of characters, settings, and themes. Blade Runner is about a bounty hunter set in a future dystopian noir while Legend is pure fantasy. Kingdom of Heaven takes place in Jerusalem during the 12th-century crusades, but American Gangster is about a 1970s drug lord operating in Harlem. And Hannibal…
You know what, let's not talk about Hannibal. They can't all be winners, right?
About the only thing these movies have in common is that they were directed by Ridley Scott, and it shows in the exquisite attention to detail put into the film even if the film isn't his best work. (Again, Hannibal. Seriously, it's not good.)
Before Alien, Scott owned a commercial production company and had directed only one other film, a period piece titled The Duellists in which the French aristocracy get stabby with one another. Although not widely known today, the Duellists did receive a lot of positive reviews and won an award for best film at Cannes Film Festival (source).
This brings us to Alien, the film that would launch Scott's name into the mainstream. As with any film, it's difficult to say what elements the director was responsible for and which ones should be credited to the producer, writers, actors, costume designers, and so on. Still, it's safe to say that Alien wouldn't have been the genre-defining, franchise-starting film it was without Scott at the helm. Here are two examples of what we're talking about:
(1) Scott hired Mary Selway, who had worked with him on The Duellists, as the casting director. With her help, they were able to cast talented but relatively unknown actors. According to Ronald Shusett, Scott wanted "to cast every other role with the most brilliant actor I can that doesn't need help from me" so that he could "concentrate on the visual style" (source).
(2) Scott also came up with the idea to have one person design all the other human settings and have another concept artists design all the settings for the alien and planetoid. Robb Cobb came on to design the Nostromo and human technology and bonkers Swedish artist H.R. Giger was convinced to come on to design the alien and mysterious spaceship. Their designs not only gave the film its own unique feel but set the stage for the numerous sequels to follow (source).
The film's history is full of these kinds of examples. Point is: Scott made Alien, and he made it work.