"Even the video games they play are high frame-rated. You are not able to create convincing imagery with traditional 24-frames-per-second shooting or just makeup to look young."

Smith says Lee was one of the directors on his "yes" list.

"I didn't even know what the project was, and I said, 'Yes!'" he says in Shanghai.

Smith says it was difficult to play two versions of himself.

"Once you get some experience, it is hard to not know, and create naivety and innocence," he says.

Smith didn't see his digital clone until eight months after shooting, he says.

" (It was) shocking and a little bit scary to see a shot of my younger self," he recalls.