According to Yaphet Kotto, Ridley Scott told him to annoy Sigourney Weaver off-camera so that there would be tension between their characters. Kotto regrets this because he really liked Weaver.
The first day that she shot a scene involving Jones the cat, Sigourney Weaver’s skin started reacting badly. Horrified, the young actress immediately thought that she might be allergic to cats, and that it would be easier for the production to recast her instead of trying to find 4 more identical cats. As it transpired, Weaver was reacting to glycerin sprayed on her skin to make her look hot and sweaty.
The spacesuits worn by Tom Skerritt, John Hurt and Veronica Cartwright were huge, bulky items lined with nylon and with no outlets for breath or condensation. As the actors were working under hot studio lights in conditions in excess of 100 degrees, they spent most of their time passing out.
Sigourney Weaver on Ellen Ripley
Ripley is introduced as a warrant officer aboard the Nostromo, a spaceship from planet Thedus returning to Earth. Having been placed in stasis for the long journey home, the crew is awakened when the Nostromo receives a transmission of unknown origin from a nearby planetoid.