According to Yaphet Kotto, Sir Ridley Scott told him to annoy Sigourney Weaver off-camera, so that there would be genuine tension between their characters. Kotto regretted this, because he really liked Weaver.
Sir Ridley Scott stated that in casting the role of Ripley, it ultimately came down to Sigourney Weaver and Meryl Streep. The two actresses had been college mates at Yale. Ultimately, Weaver was offered the job because Streep was mourning the death of her partner John Cazale at the time of casting.
Dan O'Bannon's original draft title was "Star Beast", but he was never happy with this. It was only after re-reading his script that he noted how many times the word "alien" appeared, and realized that it was a perfect title. It works as a noun and an adjective, and surprisingly, it had never been used as a film title before.
Copywriter Barbara Gips came up with the famed tagline used in the posters and promotional materials: "In space, no one can hear you scream."
Ridley Scott: [mothers] The Nostromo's computer is named "MU-TH-UR". The incubation of the Alien has also been interpreted as a metaphor for pregnancy.