Prior to shooting, the actors had all been removed from the set except for alien-pregnant John Hurt, who was hooked up to an elaborate device that involved a spring-loaded cavity filled with rancid pigs' organs, multiple hoses to pump stage blood and the puppet for the alien itself, all crammed into a false chest that was bolted to the table. Hurt was placed underneath, creating the illusion that his neck really was attached to the body cavity.
So the rest of the cast showed up on set knowing only that the script for this particular scene read, "The thing emerges." That's it. And the only thing the cast was told about the "thing" was that "its head will move and it will have teeth."
"Hey, it does have teeth and a moving head!"
When the rest of the actors were brought in, they evidently didn't notice that the entire film crew was wearing rain gear and hiding behind plastic shields. On the first take, the false chest convulsed slightly and a tiny bit of blood popped out. This was a false start to get everyone to lean in really close for the next take. Then, boom, they were hit with the full force of an internal organ eruption. Everybody lurched backed in shock. Veronica Cartright got it the worst, with a full-on blood bukkake.
Her scream of disbelief and subsequent desperate, hysterical sobbing was all completely unrehearsed. She actually passed out before the scene was finished and they had to complete it the next day. Yaphet Kotto had to retire to his room and get his blood pressure down or he might have had a heart attack.
Because this is a horror movie, so one way or another, the black dude is going to die.
If that sounds like the grossest prank any director has ever pulled on his cast, we think we can top it ...