Of all the weird and wonderful things to have emerged from the topsy-turvy world of Stranger Things, where predatory humanoid monsters routinely escape a parallel universe ruled by a giant tornado-octopus and a telekinetic 12-year-old must prevent the small town of Hawkins, Indiana, from turning into hell on Earth, Australian actor Mr Dacre Montgomery may actually be the strangest.

If you’re a fan of the show, you’ll no doubt remember Mr Montgomery’s portrayal of Max’s bad-boy stepbrother Billy Hargrove. You know, the bodybuilding, mullet-sporting teen rebel whose first appearance in the show leads one student to remark, “Would you check out that ass – just look at it go!” But the fun ends there. In the show, he’s as dishy as he is diabolical, as magnetic as he is maniacal. A babe and a bully. A cutie and a... Well, you get the picture. In the same episode, for example, Billy convincingly threatens to run over his stepsister’s friends as punishment for a snarky retort. But it’s neither Billy’s slightly psychopathic actions nor his enviable physique that make him so memorable. It’s Mr Montgomery’s utterly unnerving portrayal of an abused teenager ready to snap that’s turned him into Australia’s hottest new export. You simply can’t take your eyes off of him.

Stranger Things wasn’t Mr Montgomery’s big break – or if it was, it wasn’t his first. Before turning the heads of students and parents alike in Hawkins, he played the lead in another nostalgic sci-fi extravaganza: 2017’s big-screen Power Rangers reboot. The film was forgettable – an uneven box-office disappointment – but critics praised a few things, including the fact that it was the first blockbuster film to feature LGBTQ+ and autistic heroes, as well as Mr Montgomery and his co-stars’ performances. Having studied acting for 10 years before landing the role (he’d wanted to be an actor ever since his mom and dad, both in the industry, had taken him on set as a child), Mr Montgomery took the opportunity seriously.

“I tried to be a sponge, to soak everything up, to really make the most of it. It was a huge learning experience in every way, shape and form,” says the 24-year-old. “I remember thinking, ‘This has been a decade in the making and I really want to step up to the plate and make the most of this, and really adapt and go with the punches and give it everything I’ve got.’”

Whatever he learnt on set must have been good. Four months after finishing the film, while he was taking time off in his hometown Perth, Hollywood came knocking again. This time, they wanted him to submit a video audition for the second season of the Netflix hit Stranger Things.

“It came up on a Thursday morning, and by Thursday evening, I’d produced this whole little short film.” That audition video – featuring a shirtless Mr Montgomery embodying the deranged Billy in front of an International Klein Blue backdrop – would later go viral online for its sheer bizarreness. According to Mr Montgomery, it’s now a part of the New York Film School’s curriculum. Within weeks of submitting it, Mr Montgomery found himself on the set, having never worked in television before, of the world’s most popular new show.

The directors the Duffer brothers had introduced his character, Billy, so that in addition to the aforementioned tornado-octopus, the protagonists had a human foil, against which to channel their frustrations. It’s a trick they learned from filmic adaptations of Mr Stephen King’s novels, where certain human characters seem as evil as the monster at the story’s centre. In Mr Montgomery’s Billy, they found such a convincing antagonist – and such a charismatic star – that it simply wasn’t enough to leave him in a supporting role.

In season three, we see Billy transform from a slightly deranged bully into a fully formed bad guy, after… – well, we don’t want to spoil things – suffice to say: something suitably strange happens to him. One minute he’s the flirty, yet dictatorial lifesaver at the local pool, shouting at kids and batting eyelashes at local moms; the next, he’s abducting and enslaving a fellow lifesaver.

“In the previous season, [Billy] was more of a bully, but now he’s really raw and evil,” Mr Montgomery says. “There’s the beginning of an exploration of a real antagonist.”

Obviously, Mr Montgomery is happy with his character’s development. “Oh, mate, I’m over the moon. This season is the pinnacle of my own creative process, of what I was allowed to do, what I was given the chance to do... I couldn’t have asked for more. It was an amazing time. Not just for me and my character, but for everyone – the stakes are so much higher, and I feel like the viewers, the people who’ve fallen in love with the show, are going to get everything they wanted out of it. They’re exploding the narrative, really picking apart every character, and that’s what I’m excited about.”

So, how did he go about creating a character so convincingly creepy that the Duffers couldn’t help turning him into a leading man (albeit one who’s a villain with a mullet)? I ask him why he’s so good at playing someone so bad, and he laughs. “I guess there’s a facet of that in all of us,” he says. “I’m just channelling it. I feel like it’s so far away from me as a person. What I’ve endeavoured to do is take a group of the maniacal incidents or characters in my life and combine them, and tried to make it as real as possible. And we improvise all day; we add lines. That’s the great thing about the Duffer brothers – it’s so collaborative. I always try to come to set with 10 things, and even if they use just one of my ridiculous ideas, I’m happy.”

Between roles, Mr Montgomery keeps busy. He runs on the mantra that “half of being freelance is learning how to kill Wednesday”, and so has begun a series of projects in his spare time to keep his hands from being idle. He’s working on a podcast series for which he narrates his own beat poetry over improvised scores by different musicians. “The other day, I got the first run-through of a composition from an Australian musician and just had a total freak out dancing around the bedroom,” he says. Additionally, he’s looking at a trademark for a new company he’d like to start, one that should, he says, “make itself known to the world over the next year”.

But film will always be his first love. He’s constantly reading scripts, watches whichever film is being released at the cinemas every Thursday with his girlfriend Ms Liv Pollock (a model, with whom he recently moved to Sydney).

But he’s also just trying to live a normal life. He wears a disguise in public so as not to draw attention to himself  – naturally, he wouldn’t tell me what the disguise is – he surfs the beaches in Sydney, and he goes travelling with his girlfriend and old friends when he can. One such trip took him to LA to shoot the images that accompany this interview. Why the horse, though? “I’d been pitching to my publicist that I really wanted to shoot with a horse, and MR PORTER sorted it out,” he says. “We shot on this ranch in Malibu canyon with this beautiful white horse called Zeus. I mean, it was fantastic!”

From talking to Mr Montgomery you get the impression that he’s going for world domination, one role, project, and business at a time. “Even after you’ve shot for months on end, when you get a week or two off, anyone who works in this industry will say the same thing – you get antsy. You don’t know what to do! If you’re not on a film set, how do you fill those 14 hours a day when you’re awake? That’s why my mind is constantly on the next thing, the next thing, the next thing... I’m hungry!”