Dan the Script Surgeon

leading from 

Alien

a) As the script went forwards through it's various transformations, Ron Cobb the concept artists took note of was the weakness of the characterisation. The crew of the spaceship to him seemed almost as empty as the astronauts of 2001 A Space Odyssey and he thought that it would make it hard for the audience to identify with these people

b) Two weeks before they started filming, David Giler left the productions, whereupon the main producer, Gordon Carroll, and Ridley Scott called Dan O'Bannon in, Ridley and everyone else was fed up with Giler and Hill's failure to make any of the promised that they said they were going to make, and so a slither of opportunity opened up.

Dan said "You know, I'll fix if if you'll let me"

With that he started making the revisions,there were two weeks of frantic mutual work between them all he wasn't allowed to put it back to the way he had been originally because that wasn't allowed but he near enough patted it together a little bit into a shape that they liked, restoring some of its coherency before turning it over to Ridley to direct. Thus Dan and Ron felt a lot better about the script because of the substantial input that they were being given. Ron Cobb thought that they were strengthening it considerably and quite well . By the time they were done, what they had on the screen was, as Dan found to be in a sense very close to the original draft.

Source quotes

1. Dan O'Bannon: And then two weeks before we started shooting, he left for mysterious reasons. He left the production, and the director called me in and there were two weeks of frantic mutual work between all of us trying to put the script into a shape that they liked. By the time we got done, it was maybe 80% of the what the original draft was. What we got on the screen was actually very close to the original draft. (Rocket's Blast Comic Collector # 148)
2. Dan O'Bannon: And finally at the last minute, I saw that everyone, including Ridley, was so fed up with Gyler and Hill’s failure to make any of the promised revisions, that they said they were gonna make, that a little slither of opportunity was created. I was standing there, I said, you know, I’ll fix it if you’ll let me and I went in and didn’t put it back to what it had been originally, politically this was not permitted, but I kind of patted it together a little bit, I gave it back some of its coherency, before turning it over to Ridley to direct.  (as reported from the interview for Alien Evolution)
3. Starburst: What kind of problems did you encounter in the film
   Rob Cobb: I think the biggest problem of the film is that the people working on it who might find themselves at home with the kind of film don't have the power, and the people who have the power don't particularly like this kind of film. It creates a strange tension. Everybody wants it to be as good as possible but there's a limit to the input we can each put into the film and you can only go so far in representing a potential audience to the director and producer. One of the problems I foresee for the film is in the weakness of the characterisation. The crew of the spaceship is almost as empty as are the astronauts of 2001: I think it's a shame because it's going to be hard for the audience to identify with these people
   
   Starburst: Do you think that might change by the time shooting has finished.
   
   Ron Cobb: It's amazing. The whole film is in a constant state of flux. Script revisions are going on every day. Things that haven't been shot are still being rewritten and that's why Dan is feeling better, because he and Ron Shusett re having substantial input into these last minute script changes. They're fixing it quite well, strengthening it considerably.(Starburst #16)