Will Smith's eyes were the key to bringing back The Fresh Prince in Gemini Man

You'd think movie stars would be used to seeing their own faces everywhere. But for Will Smith, coming face to face with his own younger self in the new movie Gemini Man was "chilling," thanks to cutting-edge effects.

In Gemini Man, set for release in the US and UK on Oct. 11, Smith plays Henry Brogan, a government assassin in his 50s who's so good at his job he can pick off a target on a moving train. But after decades on the job and 72 kills, Brogan yearns for some peace and quiet. If you've ever seen any spy movies, you know retiring is never easy. And a mysterious hitman is soon hot on Brogan's tail. But there's something strangely familiar about this young killer...

The big hook is revealed in the trailers for this action-packed thriller: the aging Brogan is chased by a 23-year-old version of himself, also played by Smith -- with the help of digital de-aging by the visual effects experts at New Zealand's Weta Digital.

At the TechCrunch Disrupt conference this week, Smith joined Ang Lee, the film's Academy Award-winning director, to talk about the technical challenges of pitting the star against himself. Weta's artists even simulated the pores on Smith's face so his wrinkles collapsed in the right place when he made different expressions, but there's more to the illusion of youth than visual effects. Lee looked through Smith's old films and TV shows including Men in Black, Independence Day and The Fresh Prince of Bel-Air, pointing out how the actor's eyes "communicated a simple essence of innocence." 

"It's really a difficult undertaking to try to play naivety," Smith told the Disrupt audience. "Once you know something, it's in your body," he said. "Like, once you have sex you just walk different, you have a different swagger... It's a difficult undertaking to take knowledge away."

Digital de-aging is the hot thing in Hollywood at the moment. The technology was used to smooth away the wrinkles of several Marvel movie stars, including Michael Douglas in Ant-Man and Samuel L Jackson in Captain Marvel. It's now taking centre stage in Gemini Man and The Irishman, which uses digital effects to show Robert De Niro at different stages in the life of a murderous mobster. But while the CGI faces may look impressive, De Niro's aging gait is hard to disguise in the forthcoming Netflix gangster movie.

The screenplay for Gemini Man, originally created by Darren Lemke, dates back to 1997 and went through several rewrites before Lee signed on to direct the film. He chose to shoot with 3D cameras at 120 frames per second, a higher frame rate than traditional filmmaking that gives much sharper images. That raised a challenge for the effects teams, however, as the high-resolution cameras meant Smith couldn't wear makeup when he was acting as his younger self.

You might not expect much depth from a shoot-'em-up blockbuster --  this is nothing like Never Let Me Go, a 2010 film adapted from Kazuo Ishiguro's dystopian novel that tugs at your heartstrings because you follow clones growing up and grappling with their identity. But Gemini Man does highlight some of the dark sides of today's technology. It raises the question of whether the ends justify the means as cloned assassins might actually save lives. And the government tracks Brogan on his globe-trotting adventures, reminding us of the everyday reality that technology is a tool for government surveillance. 

Ultimately though this is an opportunity to watch Will Smith punch himself in the face. It was the idea of creating a movie star in his youth and having the two fight it out that really attracted Lee to the film.

"That's both visually and philosophically inspiring," Lee said in an interview.