WARNING: MAJOR SPOILERS AHEAD!

When effects designer David Leroy Anderson took his first meeting with Joss Whedon and Drew Goddard about their planned horror movie spectacular The Cabin in the Woods, he wasn’t exactly optimistic. After two decades in the business working on everything from Alien Nation and Pet Sematary to Get Smart and Angels & Demons, he’d grown grudgingly accustomed to having his specialty — designing and building practical monsters and makeup — usurped by the unquenchable beast of digital imagery. “You walk into the meeting with a list of 20 things you wanted to talk about,” he says, “and you leave the meeting with two things that you get to build, and everything else goes to visual effects.”

Instead, Whedon (who co-wrote and produced the film) and Goddard (who co-wrote and directed it) pitched Anderson their story — how it was about a bunch of college kids partying for a weekend at what they thought was a secluded lakeside cabin, but in reality was a highly controlled arena for ritual slaughter that could hypothetically employ practically every movie monster in the history of horror cinema. And they wanted Anderson to make all of them.

“I felt like I was being punk’d,” Anderson says. “This is the kind of meeting that you just dream of, and they just don’t happen….My brain kind of went numb.”

Then came the hard part: Actually making all those monsters. Here, in his own words, is how he did it.

For more stories behind this year’s top TV and movie moments, click here for EW.com’s Best of 2012 (Behind the Scenes) coverage.

As told by: David Leroy Anderson

Once I got started, I had three months. That’s not very much time. Toward the end of those three months, too, I was going back and forth to [the set in] Vancouver. A small portion of my crew in L.A. got to go up to Vancouver, but I had to sort of shift gears and bring another whole crew of Canadians up to speed to receive the things that were being built in L.A.

My personal favorite was the ballerina girl, the little screaming ballerina girl [with a giant mouth for a face]. I actually got to do that makeup. I drew that makeup. I got to sculpt that makeup. I kind of had to do it all on my own time on weekends. The little girl was such a sweetheart, too. I got to know her and her dad. She had a great time and was a beautiful dancer. I think she walked away very proud because she got such a terrific response from the whole crew.

She was, like, nine at the time. I’ve done a lot of horror films. I’ve worked with a lot of kids. And I have two kids. I just feel really responsible for them when they’re on set. I don’t want to scar them. So sometimes I take the parent role and continue that charade and make it a game, just kind of keep them out of the reality of what it is. Just let them do their work and hit their marks. Then hopefully they walk away and don’t need 20 years of therapy after that.

The whole Buckner family [of pain-worshipping redneck zombies] was a tremendous challenge, and I’m very proud of how the whole family came out. Unfortunately, a lot of the detail of the work we did you don’t really see in the film. A lot of the coverage of the Buckners is very dark. But on the DVD, I was really, really happy because you see a lot. When I watched the behind the scenes [doc], it dawned on me that the best chemistry on the whole film was the group of makeup artists and the group of actors who played the Buckners. They spent the most time in the [makeup] trailer. We got to know them better than anybody. It was a warm, fuzzy feeling surrounding the Buckners. They were all a delight.

NEXT PAGE: Casting the perfect buzzsaw man, and the most challenging monster of the entire movie

Image zoom Lewis Jacobs/NBC

We did everything we could to not [actively reference makeup in other horror films]. Our buzzsaw guy was a reference to the pinhead character from Hellraiser, an homage to that. We never had a picture of anything from Hellraiser up in the shop. We all kind of knew in our minds what he was talking about when Drew was explaining who this monster was a tribute to.

That particular actor who got that part [Gregory Zach] was the third person that we had considered. The first one that we did a whole round of makeup tests on — it didn’t work. Something was off. It’s a non-speaking role. This performer’s gotta bring it just with his eyes. So we cast another guy, and we did another round of tests. And it was closer. We were getting somewhere. But it was just not that scary. And while that guy was in makeup in the trailer, there was a knock at the door. Production had sent me another performer. And the door opened, and everybody in the trailer knew it. The makeup artist who was doing it knew it. The poor guy who was sitting in the makeup chair knew it. What walked through that door was buzzsaw man. This guy was so scary-looking already. He just walked in the room, and everybody was like, “Oh s—.” And I felt so bad for the guy in the chair getting the makeup because he had this look on his face like, “It doesn’t matter what I do. I’ll never look that scary.”

The merman, for a number of reasons, was the most challenging. I think we did more renditions of that character than any other character. We kept missing our cut-off deadlines. We just kept struggling to get the final approval.

Drew and Joss, between the two of them, had very clear ideas in their mind of what all of these characters looked like. Some of them were easy slam-dunks, and some of them — we’d show them something, and that helped them realize that’s not what they wanted. I know why Drew was placing so much emphasis on [the merman] — he was the big pay-off. There was just a lot more pressure on that one design, and therefore we worked longer and harder.

In one of the meetings, scratching for ideas, I think I said, “Well, how ’bout a blowhole on this thing? We could have the bloody blowhole that spurts chunks out?” And that was it. It became the bloody blowhole.

For the performer on set, it was definitely the most painful makeup. He was completely immobile. He was basically a fish for 12 hours and had to be carried around on a stretcher. When he was laying on the floor, we’d give him a little pillow, and he’d kind of curl up in a fetal position and go to sleep. There’s a lot of really cute pictures of the merman napping. We’d go gently wake him up and say, “It’s time to kill.”

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