It's one of the most iconic scenes in horror. After exploring the alien planet and having a member of the crew attacked by a facehugger, the crew gathers to have breakfast. Then all hell breaks loose. You've probably seen that sequence. Actor John Hurt thrashes around the table and eventually an alien baby burst his chest open to the surprise and horror of the rest of the crew. Did you know most of those reactions were real?
Most of the actors were kept in the dark. The only 2 people who knew were John Hurt (who had to be prepped for the scene), and the actor who played Dallas. The cast was kept away from the set for 4 hours before the scene was shot. They recall walking onto the set and they were shocked to see the crew in raingear and with buckets in hand.
Perhaps the most famous reaction is Veronica Cartwright's: when the alien pops out, she gets sprayed in blood and her scared reaction is very natural. In fact, not only was it natural and real, she passed out!