“K, being a Blade Runner, the only time he has been touched, it's with violence,” explains Blade Runner 2049 director Denis Villeneuve, gesturing with an energy not matched by his soft Quebecois drawl. “He's never had real affection, he cannot be in contact with real human beings because they don't like Replicants. And other Replicants don't like Blade Runners because they are killing their own kind.

“So he's alone - the only being he can have a relationship with is Joi, and he can't have contact with her, physical contact, so it's a very limited relationship. In fact, it's a strange, profound relationship, but it's not complete.”

Around halfway through the movie, Villeneuve and his screenwriters, Hampton Fancher and Michael Green, find a way for K (Ryan Gosling) to complete it.

Joi (Ana de Armas) - his holographic AI girlfriend - enlists the help of pleasure Replicant Mariette (Mackenzie Davis) as a surrogate body, around which Joi wraps her image. The “Merge” scene, as the film’s creators call it, is equal parts beautiful and eerie in a way that can’t quite be pinned down - a microcosm of the movie’s own paranoid presentation of what it is (or isn’t) to be human.

“ The “Merge” scene is equal parts beautiful and eerie.

“It was important for me to put the actors at the centre of the process,” says Villeneuve, “so I'd be able to find all the little elements I wanted in the scene: that we feel how K feels as he's going through that strange moment, how Joi feels for the first time like a real lover, and how Mariette is a kind of collateral victim of that love.”

To capture that range of emotions, the only major CGI work was in how the two actresses’ images were layered onto one another - up until that point, incredibly, this is a practical effect. Created without the use of green screen or motion capture, the set was instead rigged with 4 extra cameras, which were later used to interpret the room and the actors inside it as a 3D model.

“This was never going to be a scene where we would capture performances with dots away from the main set”, explains VFX supervisor Paul Lambert, whose company Double Negative won a BAFTA for its work on the film. “This allowed the actors to act without having to worry about the VFX aspect of performing separately from each other. This was the key to keeping the intimacy of the scene.”

The scene was run through with de Armas and Davis, each choosing which of their two characters would be ‘leading’ the movements of this chimeric lover as the scene progressed. Leading was the easy part.

Per Villeneuve: “The second actress would come and do the same movements again, very precisely, while keeping her own emotional arc throughout the scene.” The precision is astonishing - not least because you realise while watching that the second actress often wasn’t attempting to recreate the other’s movements, but perform them just late enough to be perceptible to the viewer, creating a realistic effect of one woman leading the other.

3D models of both actresses were created using the multi-camera footage, allowing for the more complex visual feats on show to be performed, and small movement tweaks to be made - but Villeneuve was adamant that the scene be “a merge between real photography and CGI”.

“ At one point, Joi was made of glass. Denis didn't like the look of that.

The final effect - a photo-realistic model made uncanny through translucency and glitch effects - in turn allowed for Joi and K’s love scene to achieve the effect Villeneuve wanted:

“I wanted it not to be about the visual stunt, but to be more about the emotion that the characters were going through, so it would be powerful and meaningful. I wanted it to have a kind of analog quality - that it could be a photographic process, not Star Wars-like, digital. I wanted it to be as organic as possible.”

According to Lambert, there’s a deep-rooted reason for that in the lore of Blade Runner’s world: “The digital revolution never happened in the world of Blade Runner 2049”, he explains. “Analog technologies continued to advance - the idea of pixels, digital compression, LCD, that kind of thing never came about. For the merge we based the look on the simple idea of a traditional photographic double exposure but with the complexity of being able to creatively alter the transparency in a 3-dimensional space. A simple visual but incredibly complex to pull off.”

Double exposure is a photographic technique that allows one image to be superimposed over the other on the same film, creating a new image in the process. “And from that double exposure,” adds Villeneuve, “from the merge of both women will appear a third woman, a third being. And that being will be very erotic, attractive and frightening - because it's a kind of monster, you know?”

“ The third woman will be very erotic, attractive and frightening - because it's a kind of monster, you know?

“I would say the merge scene is somewhat unique”, agrees Lambert. “We didn’t want any shot to feel like an obvious effects shot. For [viewers] to say that the scene felt so intimate means that we accomplished what we set out for.”

And what about Villeneuve? The exacting double-filming, the lack of digital shortcuts, the year of VFX back-and-forth, all for just 5 minutes of movie - is creating the most complicated sex scene of all time worth the effort? Villeneuve laughs and says, quite simply, “I'm really proud of it.”

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Joe Skrebels is IGN's UK News Editor, and those gifs were harder work than they should have been. Follow him on Twitter.