"As I walked on set," said Weaver later, "I should have noticed everyone [on the crew] was wearing raincoats."
The scene had not been rehearsed. Then Hurt starts screaming. "A small head the size of a man's fist pushes out," reads the script. Contrary to myth, it was not a total surprise to the actors. They had been told to expect something, but no specifics were mentioned. Scott - manning the camera as he did for 80 per cent of the film - was especially delighted by Veronica Cartwright's genuine expression of horror.
The designer of the chest-burster, Roger Dicken, had originally wanted a gorier scene - with the pseudo-foetal alien clawing its way out with its own little hands.
The scene has proved gold dust for parodists - not least for Hurt himself, who reprised it for Mel Brooks in Spaceballs in 1987.