Alien

Ripley, the warrant officer aboard the Nostromo. Meryl Streep was considered for the role, but she was not contacted as her partner John Cazale had recently died. Helen Mirren also auditioned. Weaver, who had Broadway experience but was relatively unknown in film, impressed Scott, Giler, and Hill with her audition. She was the last actor to be cast for the film and performed most of her screen tests in-studio as the sets were being built. The role of Ripley was Weaver's first leading role in a motion picture and earned her nominations for a Saturn Award for Best Actress and a BAFTA award for Most Promising Newcomer to Leading Film Role.

Sigourney Weaver

Dallas, captain of the Nostromo. Skerritt had been approached early in the film's development, but declined as it did not yet have a director and had a very low budget. Later, when Scott was attached as director and the budget had been doubled, Skerritt accepted the role.

Tom Skerritt

Kane, the executive officer who becomes the host for the alien. Hurt was Scott's first choice for the role, but he was contracted on a film in South Africa during Aliens filming dates, so Jon Finch was cast as Kane, instead. However, Finch became ill during the first day of shooting and was diagnosed with type 1 diabetes, which had also exacerbated a case of bronchitis. Hurt was in London by this time, his South African project having fallen through, and he quickly replaced Finch. His performance earned him a nomination for a BAFTA Award for Best Actor in a Supporting Role.

John Hurt

Lambert, the Nostromos navigator. Cartwright had experience in horror and science-fiction films, having acted as a child in The Birds (1963), and more recently in Invasion of the Body Snatchers (1978). She originally read for the role of Ripley and was not informed that she had instead been cast as Lambert until she arrived in London for wardrobe. She disliked the character's emotional weakness, but nevertheless accepted the role: "They convinced me that I was the audience's fears; I was a reflection of what the audience is feeling." Cartwright won a Saturn Award for Best Supporting Actress for her performance.

Veronica Cartwright

Brett, the engineering technician. Stanton's first words to Scott during his audition were, "I don't like sci fi or monster movies". Scott was amused, and convinced Stanton to take the role after reassuring him that Alien would actually be a thriller more akin to Ten Little Indians.

Harry Dean Stanton

Ash, the ship's science officer who is revealed to be an android.

Ian Holm

Parker, the chief engineer. Kotto was sent a script off the back of his recent success as villain Dr. Kananga in the James Bond film Live and Let Die (1973), and said he rejected a lucrative film offer in the hope of being cast in Alien.

Yaphet Kotto

the alien. Badejo, a 26-year-old design student, was discovered in a bar by a member of the casting team, who put him in touch with Scott. Scott believed that Badejo, at — inside the costume — and with a slender frame, could portray the alien and look as if his arms and legs were too long to be real, creating the illusion that a human being could not possibly be inside the costume. Stuntmen Eddie Powell and Roy Scammell also portrayed the alien in some scenes.

Bolaji Badejo

the voice of Mother, the Nostromo computer.

Helen Horton