We believe it was Frank Capra who said, and we're paraphrasing here, "If you want to film someone looking genuinely terrified, you have to scare the shit out of them. If you want your actors to cry, make them sad. And above all else, never tell them what's going on."
Directors have been taking those wise words to heart ever since. After all, why trust your actors' "training" or "decades of acting experience" when you can just subject them to actual trauma and turn on the camera? That's how we got scenes like ...
The Chest-Burster Scene from Alien
Previously on Alien, an astronaut was face-raped by an alien. It was tough going, but he recovers, until in the middle of a meal he starts convulsing like a doll being shaken by an invisible giant toddler. The crew is just about to give him space CPR when suddenly, a penis-headed alien punches through his chest and everyone is spattered with an eruption of blood and organs.
What sells that scene isn't the creature effects, or the fact that if you were an audience in 1979, it was the absolute last freaking thing you expected to happen right then. No, it's the apeshit reaction of the crew when the little bastard comes ripping out.
If you think the other actors' horrified and shocked reactions were convincing, it's probably because they, like their characters, had absolutely no idea what was about to occur.
"Yeah, I'm looking forward to many more long days of shooting."
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!"