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!"