The special edition of Close Encounters of the Third Kind came out when I was in London, by myself, at the age of 16, during a one-week break from boarding school. This was in the era of Jaws, when Spielberg was making movies that weren't so iconic that they lost all humanity. Close Encounters felt very grounded.

It's about Roy, an ordinary guy with a wife and kids, who sees an extraterrestrial. He becomes a sort of madman, alienating everybody, then meets a woman who's had a similar experience – her son's been abducted. Eventually they both go to where the aliens are having their great meet-up with humans. She gets her son back while Roy goes on the ship, never to return, at least not in her lifetime.

More than anything, seeing that film was a germ that opened my mind: the idea that Roy was going to leave Earth and travel through space, and that when he came back it would be several decades later and everybody he had known would be dead, hit home the reality of being human. It made me consider what we are, what we can be, what our limitations are. That blew the brains out of my head and I wore them on my shoulders as epaulettes.

I became obsessed with the film. I went to see it the next day and the day after that, and then I sat through it three times. Eventually the cinema staff said, "You're gonna have to leave." I had turned it into a sort of religious ritual . When I got back to school I told my best friend what had happened and he handed me a copy of Nausea by Jean-Paul Sartre. I realised, "Oh! Other people have gone through this!" Basically, the film had made me an existentialist.

There are many films and TV shows I make where people find themselves in fantastical situations; as often as possible their reactions to it are very normal. Part of the glory of Close Encounters is that Roy has to go and explain what he's seen to people, and of course they think he's a nutjob. That makes the impossible situation much, much more interesting.

I haven't seen it since that week. I keep almost watching it and then realising I just can't. It's just too important an experience in my life.

The Cabin in the Woods, written by Joss Whedon, is out now