You haven’t seen nerves until you’ve met Ang Lee on the day his new film receives its world premiere. This is Gemini Man, a frantic thriller in which Will Smith plays an assassin hunted by his own younger clone; where there’s a Will, there’s another Will, you might say. Parts of the picture were shot in Budapest, and it is here that the 64-year-old film-maker shuffles into a hotel suite overlooking the Danube. “Everything feels harder than you can imagine right now,” he sighs, sinking into an armchair. He picks up a glass from the table in front of him, then puts it down again. “Even lifting that was hard.”

He doesn’t carry himself today like one of the most celebrated film-makers of all time, a man who has never won a major award without going on to make it part of a matching pair. He has twice beaten Steven Spielberg to the best director Oscar, first for his gay love story Brokeback Mountain and then for the CGI fantasy Life of Pi; receiving the prize for the latter, he gave thanks to “the movie god”. He also has two Golden Globes and two Baftas – for Brokeback and Crouching Tiger, Hidden Dragon, his ground-breaking, treetop-scaling martial arts adventure. His movies have twice scooped the top prize at Berlin (for his Taiwanese-American comedy of manners The Wedding Banquet and his English-language breakthrough Sense and Sensibility) and Venice (Brokeback and the erotic thriller Lust, Caution).

A less ambitious director might have been content with the challenges posed by making an action movie as complex as Gemini Man, which doesn’t merely place Smith in the same scenes as his smoothly de-aged computer-generated counterpart, but shows them locked in hand-to-hand combat, and pursuing one another on motorbikes through the streets of Cartagena. Lee also shot the film in 4K digital 3D at an advanced frame rate – 120 frames a second, as opposed to the customary 24 – which gives the footage an immersive hyper-real quality halfway between an Imax spectacular and a live episode of a daytime soap. For action sequences, the technology provides an adrenaline kick. In more intimate scenes, it is every bit as eerie and exposing as it was in Lee’s unloved Iraq war drama, Billy Lynn’s Long Halftime Walk, which had the look of a hugely expensive school play.

“I was proud of that film,” he says. “But I got beat and that’s hard to swallow. I want to figure it out this time. I want to make it work and prove I was right, like anybody would. The higher frame rate in 3D is a first-person experience because we have two eyes scanning and they come to a point, so the z-axis in relation to the frame is …” He talks at length about the technical issues, insisting that audiences have been “brainwashed” by the conventional frame rate, but the more he tries to persuade me that 120fps is the future, the wearier he sounds. The impression is of someone who has sat their exams, the test paper now far beyond the reach of any more tinkering or rewriting, and is desperately trying to bring about an A-grade through sheer force of will.

I steer toward the other elements of Gemini Man, especially the way that the tension between the ageing assassin and his younger iteration seems to comment on the lifecycle of the average action hero, who will always be taunted by his younger, fitter, more handsome self. (A fellow assassin even tells Smith: “I’m a big fan of your work.”) It was precisely this idea that exercised Lee when he was offered the script, which had been kicking around Hollywood for 20 years. “It clicked philosophically for me. A movie star can always see his past on screen. What does that do to him? I knew the film could visualise that internal struggle, but it would need to be an action star who had been at the top of his game for 30 years. There are only two: Tom Cruise and Will.” He smiles. “And Tom was busy.”

There was the additional irony that Smith had made a public appeal to Lee during a promotional visit to Taiwan in 2013. The director brightens visibly when he recalls this: “In front of the press, Will said: ‘Ang Lee, I’m here in your home country! Use me before I get old!’” The challenges of shaving 30-odd years from Smith’s face were taken care of by the New Zealand special effects company Weta. With the exception of a final sunlit scene, in which the actor appears to have recently disembarked from the Polar Express, it is a convincing facsimile. Did it make Lee regret the traditional ageing makeup he had used on the cast of Brokeback Mountain? “I give a lot of credit to the actors there. Look at Heath Ledger – your heart races when you see him. I don’t know how he played that at such a young age.”

Facebook Twitter Pinterest Jake Gyllenhaal and Heath Ledger in Brokeback Mountain. Photograph: Allstar/Focus Features/Sportsphoto Ltd/Allstar

While Weta took care of the pixels, it was down to Lee to help Smith re-enter his youthful headspace. “I did some sense-memory work with him about his upbringing – he had a military father, just like the character, so we dug into that. I needed to prod him from take to take, supplying different references, keeping him alive. If you give the same direction all the time, people fall into doing the acting thing and it becomes a drill. So I used a lot of what Will had told me about himself. I learned on Life of Pi that directing in 3D is different to 2D. The same things don’t work. Because your eyes process things differently when they scan the frame and … ” Before I know it, we’re on to the z-axis again, and the joy he had when he was discussing actors is suddenly nowhere to be seen.

“Honestly, you spend 90% of your energies worrying about this stuff,” he says. “On Billy Lynn, the lead actor, Joe Alwyn, was really good because I spent all my time on him. But what about everyone else? In a higher frame rate, you can see every extra. How do you direct 400 people? With that level of clarity, you can see one guy overacting 50 yards away.”

The hostile reaction to that picture still smarts, but it’s not as if Lee hadn’t survived a thrashing before. Coming off the unanimous acclaim of Crouching Tiger, Hidden Dragon, he strode headlong into his first blockbuster, Hulk, like a man walking into a lamp-post. This was in the days before the larky, in-jokey, all-pals-together Marvel Cinematic Universe. “The first Spider-Man came out while I was making Hulk. And here I was shooting psychodrama! Back then the system was not as strict as it is today. After Crouching Tiger, they must have thought: ‘Maybe this guy can do anything.’ Hulk was the one time I had absolute freedom, which may be good or bad. Whatever I wanted, at any expense, was mine. It was like I was on a shopping spree. Anything goes! I’m still proud of Hulk, but I underestimated the power of genre and how you have to wrestle with a general audience.”

I ask what he makes of Hulk’s subsequent movie outings. “I’ve watched them on airplanes,” he says sheepishly. “Sometimes with the sound down. I didn’t really care that much. To do that kind of movie, you have to coat it with artifice. I didn’t do that with my Hulk. I went at it straight, as though it was real. With Gemini Man, I’m more aware of movie culture, more respectful of its power. When we are in a cinema, this collective imagination is like a religion. You can’t explain it. That’s the part of moviemaking that humbles you. You don’t always get your way.”

Facebook Twitter Pinterest Chow Yun-Fat and Zhang Ziyi in Crouching Tiger, Hidden Dragon. Photograph: Allstar/Sony/Sportsphoto Ltd

Lee has so many rich and varied films behind him, most of them rightly celebrated (Sense and Sensibility, The Ice Storm, Crouching Tiger, Brokeback Mountain) and others still stubbornly underrated, such as Ride with the Devil, a US civil war movie that goes off on a rewarding tangent. Could it be that his best work still lies ahead? “I would like to think so. My body will tell me. I’m turning senior citizen soon. Sixty-five! It’s hard to keep that creativity going. Energy and stamina and freshness relate to youth. I use the material to force me in new directions. My first thought when I saw this technology was: ‘I wish I was 20 years younger.’ I know I’m gonna get beat up. But I have to keep trying. I’m carrying a torch for the next level of film-makers.” He looks to the ceiling, smiles faintly and makes one last plea to the movie god: “Why me?”

Gemini Man is released on 11 October