When it comes to David and Nathan Zellner’s latest oddball feature, there are no words – quite literally, in fact. While Stanley Kubrick may have seemed daring for the 15-minute ape sequence in 2001: A Space Odyssey, the Zellner brothers push it further with Sasquatch Sunset, an entire movie in which there’s no dialogue or human activity, just four primates grunting and groaning until it’s time for the next scene. Even more impressive is that the drama – yes, it is a drama, albeit one with gross-out humour – pulls at your heartstrings, in between shots of the creatures vomiting, shitting, and every other bodily function imaginable.

“There’s so much behaviour that humans place no judgement on when it’s their dog or cat,” says David Zellner, 50, the slightly older sibling. “But as soon as it’s a humanlike creature doing similar things, it becomes uncomfortable because we have to reconcile it with the fact that we’re animals as well. What separates humans from the animal kingdom is that we have shame.”

“We were influenced by nature films from the 60s and 70s where it’s following a herd of elephants or a pack of wolves over a year,” says Nathan Zellner, 48, who co-stars as an ape. “It’s birth and death. It’s how every creature on Earth is struggling to survive. You’re seeing the inner life of a species without a voiceover. It’s immersive.”

Using real words, not grunts, I’m talking to the Zellner brothers in a London meeting room, the week Sasquatch Sunset hits cinemas. The duo, who co-directed their ape odyssey from a script written by David, know that their love/hate project will cause walk-outs, but only after their starry cast have inspired walk-ins. Beneath the prosthetics and fake fur, Riley Keough embodies the sole female character, a mother whose two mating options are an alpha male played by Nathan, and a sullener back-up depicted by Jesse Eisenberg. Trailing behind is Christophe Zajac-Denek as the child.

“We could have cast stunt performers,” says Nathan. “But to carry the emotional side, we wanted to cast against type. Jesse has Jesse mannerisms that sneak through.” In fact, Eisenberg is recognisable through his jittery eyeball movement (imagine Eisenberg’s Mark Zuckerberg coding at a computer, now picture him in an ape costume), while Keough’s forlorn stare brings to mind her solemn gaze in Mad Max: Fury Road. “It’s almost like silent film acting,” says David. “Riley’s eyes are so soulful, they convey so much information. We didn’t want contacts or VFX eyes. We needed something real to connect you to them on a human level.”

To get funding, the Zellner brothers spent years pitching the project, starting with footage of, in David’s words, “Nathan beating the ground”. Support eventually came from Square Peg, the production company founded by Ari Aster and Lars Knudsen, and Jesse Eisenberg as the first confirmed actor onboard. Once the ensemble was formed, Eisenberg brought on Lorin Eric Salm, the movement coach who taught him mime for Resistance. “Lorin worked to get everyone on the same page, as though we were creating a species,” says David. “We pulled a lot from primate behaviour, but at a certain point you make your own rules.”

Before the shoot, the actors collectively rehearsed in person and on Zoom, while at home they practised their apelike mechanics on their own. “You lead more with your shoulders as opposed to your hands,” explains Nathan. “I would get up and roll around. The goal was to get to a comfort level so that you don’t have to think about it on set, and you can just think about the emotional side.”

Taking place across a year, Sasquatch Sunset unfolds like a three-act film – or four, given how it’s divided into seasons. Fuelling the action, the characters are driven by pure instinct: love, sex, food, and, of course, being so jealous of someone else that you’re prepared to ruin your life as well as theirs. “Structurally, we wanted it framed around four seasons so that it’s not a timetable imposed by humans,” says David. “We never wanted voiceover. As soon as there’s a voiceover, there’s a human judgement. We wanted it to be in this Bigfoot world.”

Shot with natural light in actual forests, the film inadvertently emulates a nature documentary, except one lensed by Mike Gioulakis, the cinematographer behind the iconic beach scenes in Us and It Follows. “A lot of times, with creature work, it’s done on a soundstage or a controlled environment,” says Nathan. “But we wanted to shoot on location in a very primordial setting with real animals. Every element outside of the characters has a real, naturalistic quality, and the natural light normalises them.”

“It’s birth and death. It’s how every creature on Earth is struggling to survive. You’re seeing the inner life of a species without a voiceover. It’s immersive” – Nathan Zellner

As a filmmaking duo, the Zellner brothers have specialised in love-it-or-hate-it projects. After all, their breakthrough was in 2015 with Kumiko, the Treasure Hunter, a cruel comedy about a Japanese woman who mistakenly believes Fargo – primarily, the scene with the buried money – to be a true story. A few years later, their 2018 western, Damsel, was wholly unlike its marketing: viewers expected an epic drama with Robert Pattinson and Mia Wasikowska; instead, it was a screwball farce in which Pattinson dies early on. Recently, they directed three episodes of Nathan Fielder and Benny Safdie’s divisive TV series The Curse.

When I suggest that the faux-non-fiction vibe of Sasquatch Sunset matches up with the voyeuristic mise-en-scène of The Curse, David responds, “Oh, maybe. I don’t know. They had seen Kumiko, and we’ve known Benny from film festivals.”

“We observe things,” says Nathan. “One of the things we were drawn to in The Curse was their approach to observing the characters, as opposed to being right there.”

“TV is a different dynamic,” says David. “[Fielder and Safdie] had a very clear vision of what they wanted. We were helping to service their vision, and luckily it’s similar to our sensibilities. With TV, when the creator of the show is also the star, they know what they want, and it’s helping to facilitate that.”

During the pandemic, the Zellner brothers developed a fully animated Looney Tunes movie for Warner Bros, only for it to fall apart. They cite Looney Tunes as a key influence on their entire career and, in turn, Sasquatch Sunset. “There’s the birthing scene, when he’s lining up to catch the baby,” recalls Nathan, laughing. “And he’s 20ft away. That ridiculousness is very Looney Tunes-esque.”

Somehow, Sasquatch Sunset is the Zellners’ first film to receive a theatrical release in the UK, which means that a year after seeing a big foot on the big screen in Barbie, you can see a Bigfoot on the big screen yet again. On a hot streak, the Zellners are preparing to shoot Alpha Gang, a sci-fi comedy starring Cate Blanchett about extraterrestrials taking over planet Earth. Suffice to say, it’ll be wholly unique within their filmography, if not the movie landscape at large.

“For better or worse, we gravitate towards things that have an element of risk,” says David. “That’s what interests us with art in general.” That’s all evident in Sasquatch Sunset: even if the diarrhoea, pissing, and finger-sniffing sequences are played for laughs, they also exist on a philosophical level. “We knew there’d be a certain level of humour,” David continues. “But the real challenge was for it to have a level of poignancy and relatability. That was the most important thing to us.”

Sasquatch Sunset is out in UK cinemas on June 14