I’ve always been fascinated by why people do what they do,” says Joe Berlinger. We’re sitting on a sofa in a brightly lit London hotel room, but darkness pervades our conversation. Years ago, when we talked about Book of Shadows: Blair Witch 2, his radical sequel to the indie blockbuster, The Blair Witch Project, the Connecticut-born film-maker told me he had “spent a lot of time staring into the abyss of real evil.” Almost two decades later, Berlinger, laughing, says, “I guess I still am.”

He isn’t kidding. His latest subject, the notorious serial killer Ted Bundy, who confessed to committing 30 homicides between 1974 and 1978 (the true tally could be higher), just days before being executed in 1989, is such a study in perversity, that immersing himself in the murderer’s life even shook Berlinger, a leading exponent of true crime documentary making, “to the core, as to my belief in human nature and the capacity for evil.”

Berlinger has had two cracks at the Bundy story. His first, a four-part documentary series, Conversations with a Killer: The Ted Bundy Tapes, is currently streaming on Netflix. In it, he uses a journalist’s taped interviews with the killer, old TV news reports, and new interviews with law enforcement agents who hunted him, people who knew him, lawyers, and a survivor of an attack by him, to try and get inside Bundy’s mind.

The director’s second take on the story, Extremely Wicked, Shockingly Evil and Vile, sees Berlinger returning to feature film-making for the first time since Blair Witch 2, whose hijacking by the studio led him to disown the movie before its release in 2000.

Zac Efron, the young Jewish star best known for Disney’s family-friendly High School Musical franchise, makes a surprisingly effective Bundy. However, don’t expect to see him covered in blood— the film is told from the point of view of Elizabeth Kloepfer (Lily Collins), a single mother who lived with Bundy for years, unaware of his dual nature, and only reveals the deadly truth about the man in its final moments.

Berlinger creates an alarming sense of cognitive dissonance as he draws us into their relationship, making us root for them while knowing what really lies behind Bundy’s good looks, charm, and claims of innocence. Even the splashily lurid title, which almost makes the film sound like a gleefully trashy John Waters-style comedy, turns out to be something altogether darker.

For the film-maker, Bundy is a warning. “We want to think a serial killer is some twisted outcast, a misfit who doesn’t fit into society, because that gives us the false comfort that you can spot them a mile away, and therefore, potentially, avoid a bad fate, but Bundy teaches us the opposite,” he argues. “He fitted in very well with society. People liked him. He was a wonderful surrogate father to Liz’s daughter from another relationship.”

Evil doesn’t announce itself, then, and is often embodied by “those closest to us, the people you least expect. Whether it is a priest who commits paedophilia and holds Mass the next day, or whether it’s like Ted Bundy, that’s the true nature of evil.”

Berlinger has two daughters in their early 20s, “the prototypical Bundy-victim age”, who knew nothing about the serial killer or the trail of misery he carved across multiple US states. He wants them, and others like them, to realise: “Just because somebody looks and acts a certain way, they don’t necessarily deserve our trust . . . That’s a lesson you can’t state enough to young people.”

And it is a lesson that is becoming more urgent, he appears to suggest, citing the ongoing case of a fake Uber driver who allegedly kidnapped and murdered a woman in Carolina. Meanwhile, the internet has created a new hunting ground, full of smoke and mirrors.

“People curate an idealised version of themselves on social media. I’m guilty of it, too,” Berlinger admits. “If you look at my Instagram you’d think I’m every day having just premieres and wonderful times. But there are some dark days, too, like everybody.” He’s “not knocking” anyone smoothing the rough edges off their lives online, he insists, but stresses that “on the other end of that spectrum, there’s more of a nefarious intent and abuse of pretending you’re one thing when you’re really not.”

Efron’s casting makes perfect sense in the context of Berlinger’s thematic designs. He was the director’s first choice, partly because of acting skills which Berlinger didn’t think the handsome 31-year-old had been given an opportunity to fully show on screen, but also because of Efron’s real-life “teen heartthrob persona”.

“For a certain demographic, he’s a guy who can do no wrong because of how he looks and acts,” says Berlinger. “To me, as a documentarian, it felt like something authentic that I could take into this exploration of deception and betrayal.”

It required “a number of serious conversations” to convince Efron, however, who needed to know that there wasn’t any risk of glamorising Bundy, and that his participation wouldn’t look like a stunt, or some desperate bid to go dark. Ultimately, he believed in the film’s message, and agreed to a “massive pay cut” in line with the film’s “very low budget”.

“So he was really committed to doing this for all the right reasons,” says Berlinger. “The fact that he was willing to play with his persona like that was a brave thing to do because this could have totally backfired.”

As Berlinger and Efron are both Jewish, I ask if there’s a historical dimension to his fascination with evil and the mask of normality it often wears. In Blair Witch 2, the main characters are blind to their own capacity for violence. During the Holocaust, ordinary people turned on their Jewish friends and neighbours . Is Berlinger influenced at all by our tragic past?

“I would be overstating it to say that that’s what drove me to do this particular film, but [the Holocaust] did shape my world-view,” he says.

His family is of German and Jewish heritage and, while his great-great-great-great-grandfather had left Germany in the mid-1800s as a teenager, Berlinger nonetheless “started to become obsessed with the Holocaust. I had seen some Holocaust films and I thought to myself, ‘okay, I’m of German descent, and I’m Jewish, but neither of these traditions is particularly strong in my family, and yet had I been in Nazi Germany, I would have been killed.’”

This led him to major in German at university as part of a route to try and “deeply understand, how is it that Germany did this?

“So, I think my nose for social justice, my desire to use film to right some wrongs, which my documentary tradition is very much about that, and the idea that you can’t trust those around you, really grew out of my experience of studying the Holocaust.”

These days, it isn’t just people we have to be wary of trusting, though, but also information. Berlinger meditated on the dangers of blurring the line between reality and entertainment in Blair Witch 2, but seems shocked by his own prescience.

“I never could have predicted where we are today: alternative facts, fake news, people in their own media silos screaming at each other, it’s exactly what I was talking about!” The “sad, pathetic, scary part about Donald Trump is not Trump himself,” he claims, though. “It’s that 40 or 45 per cent of the country that believe in what he’s doing and what he’s saying . . . And that’s exactly the dangers I was trying to discuss in making that film.”

The idea of media and entertainment converging also connects with the Bundy story. The killer’s trial in Miami was the first to allow TV news cameras into the courtroom, and Bundy, who by this point had become a kind of gruesome celebrity, made the most of it, as did the viewing public, who consumed the trial as a sort of grisly soap opera-cum-drama.

“For the first time, Americans, by the millions were able to watch serial murder and serial rape as live entertainment, in the comfort of their living room,” says Berlinger.

“So the fascinating thing for me about Bundy — and one of the reasons I wanted to do these projects — is that Bundy, for me, is the Big Bang of our current insatiable appetite for true crime. . . It’s like an almost self-reflexive look back at what started this phenomenon that I am a participant in.”

While some true crime stories “wallow in the misery of others for entertainment”, Berlinger insists that his work always has a “social justice aspect to it, whether it’s wrongful conviction, or victim advocacy, or shining a light on some aspect of criminal justice reform.”

Famously, his acclaimed Paradise Lost documentary trilogy, co-directed with Bruce Sinofsky, helped bring about the release of three young men, known collectively as the West Memphis Three, wrongly imprisoned for murdering three children.

Ironically, the failure of Blair Witch 2 gave him time to focus on the case. It also made possible his excruciatingly candid documentary about the heavy-metal band Metallica, Some Kind of Monster.

“To me, it was actually a gift,” he reflects, “because it chased me away from scripted movies for a long time… And it’s good to have your face rubbed in the mud every once in a while. It keeps you humble.”

‘Extremely Wicked, Evil and Vile’ is in cinemas and on Sky Cinema from today