Watch Sigourney Weaver’s 1978 ‘Alien’ Screen Test.

One month before filming began on Alien, Ripley still hadn’t been cast.

When Dan O’Bannon and Ronald Shusett wrote Alien, they didn’t specify which members of the ill-fated Nostromo crew were male and which were female; all of the parts were written unisex, allowing Ridley Scott and his team the unique freedom of deciding the sex of each character during the casting process. Even Ripley, the Nostromo’s sole survivor, was not a female character until Scott came aboard the project. Of course, he cast 29-year-old Sigourney Weaver as the film’s heroine, and it’s impossible to imagine any other actor (male or female) doing the part more justice.