First, a disclaimer: SPOILERS AHEAD! Now, a second disclaimer: Don’t do it. Don’t even bother. If you think you can’t do it without blurting out some detail that would ruin the entire movie, then you really shouldn’t try. You’ll let something out about the crazy elevator of a thousand monsters. You'll let something slip about the zombie redneck torturers. How would you feel if you knew going in that it was all an elaborate sacrificial scheme dedicated to appeasing an ancient hell-god with a giant coal hand? Chances are you wouldn’t be as interested in the wolf make-out scene (Or maybe you would, but that's weird).
Still around? Okay, good. Carrying on a spoiler-free Cabin in the Woods conversation is a difficult and entertaining activity. It’s like juggling knives while snowboarding, without the need to go to a mountain and with no danger of dismemberment. First, you’ll need to arouse the interest of your friend. Come up to them and say something like, “I just saw Cabin in the Woods. It’s the best horror movie ever.” They’ll either be interested, or say something about not liking horror movies. Then add, “It’s not even a horror movie. That’s what makes it so awesome.” They’re now interested too. Whatever you do, don’t mention anything about the fact that the real plot of the movie has nothing to do with a cabin or the woods.
Next, while you’re talking, vagueness is key. There are three routes you can take with this. You can give them the short genre synopsis: “bureaucratic horror-comedy” is a current favorite. You can give them the longer summary: “It’s a movie where a group of kids go to a cabin in the woods, and nothing – and everything – is as it seems.” Or, you can make them ask you questions.
Making them ask you questions is much more fun, as you have more opportunities to be vague. If they ask you who’s in it, you respond “Thor, a former yellow Power Ranger, and more.” If they ask, who more is, simply say, “You’ll see.” Perfectly vague. Then they may ask what movies it’s like, suggesting genre classics like Friday the Thirteenth, Texas Chainsaw Massacre, or even Scream. Is it like any of these movies? Answer: maybe. Once you’re finished being vague, start being specific. Now, it would be wrong to divulge any major plot points or twists. However, there’s nothing wrong with giving them a few red herrings. Reveal interesting facts that aren’t integral to the plot. Say that it has the coolest coffee thermos ever. Mention the epic motorcycle-jump related speech. Focus on things that really don’t matter. Talk about the nondescript Winnebago, or how perfect Chris Hemsworth’s performance is. (“He brings serious gravitas in an unexpected way,” you can say) Mention that a badly rendered CG eagle has a big role. Have fun making them look for things in the movie that don’t matter. Once again, don’t mention anything about ancient gods or anything relating to them, including a compound full of people figuring out ways to kill the main characters.
Or, you could just spoil everything. Tell you friends that it’s about a crazy house on top of a prison full of monsters like werewolves, the kid from The Ring, mermen, and redneck torture zombies. Chances are they won’t believe you.