Originally the alien was to be found in the closet of the escape shuttle that would later be named the Narcissus. Ron Shusett thought that it would be a great moment if it was done right, Ripley would run, get to the safety boat, and its launched, the ship explodes and it's in the escape shuttle with her in the closet, this would have been the payoff.
After Dan O'Bannon had returned to the US, Ron and Ridley were having a chat , perhaps two thirds through the movie production. The command module of the Nostromo as seen in the opening sequence had been economically converted and redressed so that it became the interior of the Narcissus which was the escape pod.
Ridley said to Ron "You know, wouldn't it be great if we had one more surprise because you've got a chestburster, and the robots head coming off. If we could have one more amazing thing in the lifeboat"
In view of the original alien in a closet ending, Ridley said "Can't we beat that?" Then he came up with another idea "She's somewhere, he's somewhere and she is there, but it's with her, can't it be somewhere she and the audience can't see it, it's camouflaged so well, and it just emerges!
"Right, okay, so where's the alien going to be?"
They agreed "No, that's too easy because people don't look at the roof, it should be hiding in plain sight"
Besides that Ridley had already filmed the scene where the alien comes down like a bat on Brett, and wouldn't have been able to tolerate repeating himself
Ridley's view was " It should be that the audience can see it, but they don't know where they're seeing it."
Ron said "Ridley you're absolutely right"
Ridley replied, "okay, let's work on this"
Ivor asked "What if the alien is like a chameleon, and he's going into another stage? Giger's concept was biomechanoid, so if you front lit the alien, so the audience could see it, that would be an amazing coup. " *Alien 1979