Terry Gilliam managed to see the Alien, and because of the way he talks about it, he might have seen the preview at Shepperton studios since he was around at the time making the Monty Python movie "Life of Brian".
While he watched , he saw it as another person who saw it as a ghost train where something jumps out and viewer doesn't know who's going to die next
When he first watched the first Alien, he found himself saying "Just kill them all and be done with it," because he knew that they were all going to die on the way.
In the end he saw he igourney Weaver, who has been established is a really tough military officer, is running around in her underwear trying to find a cat. Although of course she wasn't really running around in her underwear and perhaps this confused memory would form many decades later in his mind, she stripped down to her underwear briefly in the last scene. But the idea of going to save the cat didn't seem to interest him anyway
He thought that there were some great shots in the film but one shot that should have never been in there was the one where the alien is getting blown out of the airlock because it showed that the creature was just a man in a suit. Up until then, the viewer would only see bits of the alien, making it seem huge, vast and terrifying, and that he could be compared to the shark in Jaws.
He told Ridley, "You don't want that shot of the alien at the end. Cut it!'"
- "'Alien' is just a ghost train where something jumps out and you don't know who's going to die next. When I watched the first 'Alien,' all I kept saying was, 'Just kill them all and be done with it,' because you just know that they're all going to die along the way. In the end, Sigourney Weaver, who we've established is a really tough military officer, is running around in her underwear trying to find a cat. Give me a f-king break. There are some great moments in it, but the shot that should've never been in the film is the one at the end showing the alien getting blown out of the airlock. You see the alien, and it's just a guy in a rubber suit. Up until then, you only saw bits of the alien, and it seemed to be huge and vast and terrifying. That was so clever. It was like the shark in 'Jaws.' I told Ridley, 'You don't want that shot of the alien at the end. Cut it!'" (KVIFF 2018: "Terry Gilliam Dreams the Impossible Dream" by Matt Fagerholm July 5, 2018 https://www.rogerebert.com/interviews/kviff-2018-terry-gilliam-dreams-the-impossible-dream)