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MamaRAMa
I remember reading that the actors did NOT know what was going to happen in "the chest" scene. The director wanted their reactions to be authentic so they did not know ahead of time how the scene was going to go exactly. The only actor in the scene who knew what was going to happen was the actor who was the alien's host, John Hurt.
Jumping on to add some detail. Namely, what they didn't know was that they were going to be sprayed with fake blood and ground-up cow viscera being shot at them from high-pressure pumps.
According to the producer and co-writer David Giler,
"The 'Chest Birth' was simulated for the actors by surprising them with a shower of animal entrails. That's why their looks of disgust and horror are so real. They had no idea what we were going to shoot that day."