Alien: List of actresses for Ripley

leading from 

Alien: Sigourney Weaver cast as Ripley

a) Laddie was going crazy saying to Ridley "you've gotta make your mind up".and Ridley's response was "yeah, I can't you know, I can't find it yet."

They started going through lists of actresses, seeing who was right for the role. 

Names such as Catherine Ross from "The Stepford Wives" and Genevieve Bujold from "Coma" were thrown around, but looking at the success of Star Wars with actors who were not famous, they thought that they could go for unknown actor to star in the movie

Meryl Streep, Katherine Ross and Genevieve Bujold

b) Mary Goldberg in the casting department told them that there were two women who they should see in New York who she thought was perfect. 

Mary Goldberg from the casting department

One a woman named Meryl Streep had done some films, but had not broken through as yet, she had been filming "The Deer Hunter" and that wouldn't be released until the end of 1978. 

However Meryl's long time partner John Cazale had died on March the 12th from bone cancer, and it was a day later that they were holding this discussion so they thought she shouldn't be asked to come in and they could see her another time. 

The other was a young girl who had not really made a movie as yet, named Sigourney Weaver.

leading to:

Alien: Hearing about Sigourney

Source Quotes

1. The studio considered hiring a highly established female star like Meryl Streep, Catherine Ross or Genevieve Bujold, but the recent release of Twentieth Century Fox's Star Wars had proven that big names weren't always necessary to make big money. Everyone thought the same strategy should also apply to Alien, so the studio launched a massive search for an unknown actress to star in the film. Eventually the choice was narrowed down to a young New York theatre actress whose most notable film experience had been a six second walk on in Woody Allen's Annie Hall (Alien Saga) 
2. Sigourney Weaver: Ridley Scott and the producers came to New York with a list of people to see that they had gotten from casting people out in Los Angeles, and I was on the list. I really hadn't done any film work, and I remember thinking that I wasn't sure that I wanted to do any science fiction - I mean, I had such a classical training, I thought I would get a nice supporting role in a movie with some "integrity" (Official Aliens movie book)
3. Mary Selway: Casting: We started going through lists of actresses, and who was right for the role.  You have to remember that Sigourney Weaver, in those days, was not a big star, and she tested for the part. (Alien Quadrilogy Documentary)
4. Ridley Scott: So that would put me, I don't know, that would put me a month or five weeks out, so Laddie was going crazy saying "you've gotta make your mind up". I said "yeah, I can't you know, I can't find it yet." (see: Ridley Scott at the Hero Complex Festival on Youtube, transcript)