His wickedly hilarious films are the stuff of nightmares but the man behind the gory Hereditary has his own terrors to conquer

Midsommar is an agonisingly lovely horror film about four American students bungling an invite to a pagan celebration in Sweden. It is highly likely to give you nightmares. The film’s writer/director, Ari Aster, is currently having one of his own: battling reporters determined to discover his own demons. “I’m cripplingly neurotic when it comes to these interviews,” he says. “For me, these are just total minefields.”

Hereditary, his breakout debut from last year about a miserable family, featured two beheadings and a sobbing Toni Collette literally climbing the walls. People sensed that this soft-spoken, charmingly awkward young film-maker might have been inspired by a past he didn’t want to share. After much grilling, Aster has learned to reveal small intimacies (a stutter when he was young, a solitary childhood) while palming the truly personal.

Hereditary was the first horror script he wrote. Midsommar – inspired by a breakup – is the second. It is wickedly hilarious, yet clarifies the impression that Aster approaches movies like a skilful butcher, flaying his own neurosis to present audiences with quivering raw meat that could be his heart – or maybe, one fears, their own.

The film opens with a college girl, Dani (Florence Pugh), losing her parents and sister to an incomprehensible tragedy. Even before this, her relationship with her longterm boyfriend, Christian (Jack Reynor), was shaky. Christian’s mates – Josh (William Jackson Harper) and Mark (Will Poulter) – pester him to break up with her; instead, he invites Dani to tag along on a boys’ holiday to their friend’s home in Hälsingland, a flower-strewn paradise with a few fatal customs.

Facebook Twitter Pinterest Aster on the set of Midsommar with Florence Pugh. Photograph: Merie Weismiller Wallace

It was Dani with whom Aster most readily identified. “I’ve certainly been in her position many times, where I am clinging to something that’s dead because I’m not done with it,” he says. Aster was eager to hone Pugh and Reynor’s hostile, unspoken dynamic. The actors were instructed to cook meals in character, as well as to field questions about their feelings as if he were their couples therapist.

As for the anthropology, Aster spent months researching Scandinavian and Germanic myths and traditions, touring museums and ancient farms and studying plants, paintings and Viking torture techniques. Drug-induced hallucinogens, gory prosthetics and a particularly ripe herring are also thrown into the mix.

Aster likes films that disturb. At four, he had his first cinematic experience, seeing Dick Tracy. When the tommy guns started blazing, he fled. “My mom had to chase me down 10 New York city blocks [while I was] just screaming my head off.”

A few years later, the family moved from Manhattan to Chester, in the UK, where his father opened a jazz club. Aster liked Chester, but at 10, the family relocated again, to New Mexico, upending, he says, his stability.

He dealt with the culture shock by diving into film. He admired blockbuster auteurs such as Tim Burton, and rhapsodises about Batman Returns. “It’s this work of bizarre Hollywood expressionism!” By 11, he had watched A Clockwork Orange and Blue Velvet, and hated them both. “They felt really mean,” he says. Yet, he watched them over and over to make sense of why they rattled in his brain. “I developed a taste for mean-spirited storytelling,” he says. He spent his youth alone, writing scripts. He couldn’t bear to make something amateur. “I knew I didn’t have the actors or the lighting equipment,” he says. “It was maybe indicative of a larger arrogance.”

My mom had to chase me down 10 New York city blocks. I was screaming my head off

Eventually he made it to graduate film school at the American Film Institute in Los Angeles, where his first film deliberately provoked the AFI’s sensible tastes. “What’s the worst thing I could do here?” he asked himself. “And I was like: ‘What about a son molesting his father?’ Nobody should do that. So then it became an exercise – can I make that compelling?”

He could. The Strange Thing About the Johnsons, a 30-minute incest melodrama about a twenty-something man who, even after his own wedding, continues to abuse his dad, is stunningly confident film-making. And if viewers listen closely, they can hear Aster’s voice as a motivational speaker who warns: “Horrible things happening to people is inextricably linked to people thinking horrible things might happen to them.”

That muffled caution might capture his career to date. Not that he has outrun his fears. Shooting Midsommar in Budapest, Aster was so paranoid about ticks that he wore green knee-socks and a netted safari hat. “I’m a neurotic Jew and I’m afraid of Lyme disease,” he says. “It was very embarrassing, but I cared more about my peace of mind than being fashionable on the set.”

Now, he says, he’s done with horror. “Everything else I’ve written is in a different genre. I love musicals. I love romantic comedies. I would love to do animation.” Then he leans forward to show me a sketch on his phone, of what looks like a parrot beak bursting through a bashed-in head. That horror bug is hard to kill.

• Midsommar is out in the UK on 5 July