When Jan Oliver Lucks’ fiancee left him for another man he was more distraught than most: she not only walked away with his heart but the end of the relationship also meant the end of the documentary they were working on.

The pair had been collaborating for more than a year on a project documenting their open relationship, and when his partner decided to leave, the filming stopped too.

“We filmed to a point where it wasn’t healthy, I was filming too much, I wasn’t paying attention to her. Because there was that sort of filter between us – the camera. It meant everything she said or we did or discussed went through that filter and me thinking: ‘would this be good drama, would it be good for the film?’” Lucks says.

“I had my film-maker’s cap on and the phone [camera] as a shield but I was stabbing myself in the heart.”

Just after getting engaged in late 2015, the couple agreed they would be free to sleep with other people, which for Lucks meant men as well as women. Both were motivated by a curiosity about non-monogamous partnerships, and Lucks also felt he had “wasted” his 20s and university years by not sleeping around.

“At the start I just wanted to try out sexual things that I hadn’t tried out before and I felt like a sexual under-achiever,” Lucks tells the Guardian from his apartment in Auckland.

“When we switched from an open relationship to a polyamorous relationship things got very complicated … it was hard for both of us.”

Gradually, the arrangement – and the rules the couple had made around it – led to emotional entanglements for both parties, with Lucks finding a girlfriend in Dunedin and his ex meeting a man she fell in love with.

Jealousy and self-doubt flared, but as the filming continued, Lucks put aside his growing unease in pursuit of better and better footage.

“I think polyamory can work but you have to have a lot of patience, time and good communication skills,” Lucks says.

Just as the wedding invites were printed, Lucks’ fiancee called it off. While Lucks retained his half of the footage, mostly filmed selfie doco style – her half was gone, and so was their film.

An alternative to monogamy

Lucks, 37, is an established New Zealand film-maker, a product of the well-regarded Otago University natural history film-making course, and with one local hit already under his belt.

He entered a depression following the separation, thinking that not only had he lost his fiancee but also a project he had spent years on.

“It took about two weeks to come up with the idea of casting an actress,” Lucks recalls. “We had my half of the footage which I knew we could use and then recreate the other half.

“Obviously it was a difficult time – but having this idea meant I had focus again and I had space to write myself back together.”

Enter Natalie Medlock, 36; a British-born New Zealand actor, director and writer, who is mainly known for her improvisational work, and an appearance as a nurse on local soap opera Shortland Street.

With the blessing of Lucks’ ex, Medlock was brought in to recreate, reimagine and rebirth their love affair. She was also asked to be a co-writer.

“It made me really respect and want to honour both their sides of the story and protect Ollie as well and not view what he was doing as masturbatory in any way – but brave, and vulnerable,” Medlock says.

Key lines and scenes from the real romance remained unchanged in the eventual feature film There is no I in Threesome, now playing on HBO Max, as did the major plot arcs.

Lucks, who had never acted before, decided to go “method” with his performance and effectively relived the entire romance, which he says led to sleep problems, depression and a withdrawal from fresh romantic relationships.

“I felt at the time if I didn’t make a film out of it, I would have given up the love of my life for nothing and that just felt like a double defeat.”

The twist

Medlock’s true identity as a hired actor is kept hidden until the final segment of the film when viewers – many of whom assume the film is a documentary – are finally let in on the secret. The result has triggered a wave of publicity, with the New York Times calling it “an astute examination of perspective”, while the Los Angeles Times described the twist in the final act as “shocking”.

The meeting of memory, fact, fiction and storytelling intrigued Medlock.

“I was fascinated by the vulnerability at the heart of the piece. And not knowing where it was going exactly. It was really the humanity in Ollie and the story rather than the actual subject matter [of polyamory] that intrigued me.”

Medlock has never formally met Lucks’ ex and had to piece together her character through interaction with Lucks and improvisation.

“I had to really foster that relationship [with Lucks], and all of the kissing and everything was initiated by me,” Medlock says, who also credits an intimacy coach. “Because he kept disappearing behind the camera, just like he did to his ex. So that was very interesting. I had to constantly bring him back. I really felt, ‘he’s gone’ and I must have felt 2% of what his ex felt – so I think I felt a lot of empathy [for her].”

Lucks’s ex has now seen the film, and largely likes it.

Lucks describes the film as a “cautionary tale”, and says that he and his ex did everything you shouldn’t when opening up a relationship – including living apart.

Neither Lucks nor Medlock has any interest in entering an open or polyamorous relationship in the future, though Lucks remains intrigued by the possibilities of a non-monogamous lifestyle.

“I think most people think about open relationships at one point in their lives … we thought it would be interesting for others to see there’s an alternative to monogamy and we wanted to show others how well it can work,” Lucks says, with a laugh.

“It started with a lot of rules about only hooking up once and it was just about sex but slowly those rules slid away and emotions became involved. If I wasn’t filming I should have been more present and realised I couldn’t do it.”