Having the android idea

Leading from Ash the Robot

a. Giler and Hill conceive the idea
Hill and Giler introduced into the story of alien a character who was an android named Ash. They don't actually remember which of them had come up with the idea but it came as a joint decision and a cheap idea.  However as Giler remembered it, Walter Hill said, "I have what I think is a dreadful idea or a really good one. What do you think of this? Suppose , in this part, whack! his head comes off and he's a robot."

Giler replied "Well terrific, let's do that, and we'll put it on a table and  we'll have the head talk."

So they went back and made the subplot work for that, David Giler actually wanted the first words from the robot on the table to the the Kipling open" If you could keep your head about you."

b) Ron Shusett's approval
Ron Shusett loved the idea and would be eternally thankful to David Giler and Walter Hill for adding that idea. He felt it was the next thing that one needed after the shock and horror of the chestburster scene he thought it was close to being as amazing, and it kept the third act from being a let down.

c) Dan O'Bannons's disapproval 
The general idea of what constitutes a suspense story was an issue of some contention amongst the producers and Dan lost a couple of those battles. There was no Ash in his original script and Giler and Hill added it in his opinion just for the sake of a sub plot because having a single plot was by itself for them inadequate. However Dan thought that Ian Holm gave a brilliant performance and it was brilliantly directed by Ridley but Dan saw nothing so interesting about the revelation that someone was a robot.

 source quote

1. Fantastic Films: He's not an ordinary robot is he?
   
   David Giler: Walter Hill and I were writing the script - we were working the script and we had invented the subplot of this dodging character. And Hill said,  I have what I think is a dreadful idea or a really good one. What do you think of this? Suppose , in this part, whack!, his head comes off and he's a robot. " Well terrific ," I say, "let's do that. And we'll put it on a table and then we'll have the head talk." So we went back and made the subplot work for that. Actually at one time I wanted the first words from the robot on the table to be the Kipling poem, "If you could keep your head all about you..." (Fantastic Films #13, p60)
2. Ron Shusett: They contributed one thing that was one of the best things in the movie and I will eternally be thankful to them, not only for starting the ball for financing but to contributing this to the movie, what they invented was the robot, that was not in the movie,  but the robot, Ash is a robot and his head comes off, that whole idea and scenario was theirs  (Developing The Story, Alien Quadrilogy DVD) 
3. Ron Shusett: This is Giler and Hill's concept , it's the only thing that wasn't in the orginal script of Dan's and mine and the reason I knew it'd work because you needed something after the chestburster that was at least close to it as amazing and that is this, when the head is knocked off and you don't realise he's a robot and that kept the sec... the third act from being a let down. Oh god, shit, Jesus christ   ( Bluray version of the Alien Quadrilogy commentary at 1:21:10,)
4. Dan O'Bannon: "The general idea of what constitutes a suspense story was an issue of some contention amongst the producers and I lost a couple of those battles. There was no Ash in my original script, they added that, the idea being here that all...all scripts must have a sub plot, simply to have a single plot by itself is inadequate, all stories must have subplots. So they created a subplot. Ian Holm gives a brilliant performance, it's brilliantly directed by Ridley, but if you stop and think about it, if it wasn't in there what difference would it make one way or the other, who gives a rat's ass, so somebody is a robot. (Alien commentary, Quadrilogy, 01:17: 29 - 01:18:29.)