There are a couple big reasons behind that necessity, but Lee’s greatest caveat was that in such an advanced film, it’s the only way the effect would have looked believable to the audience. I was in attendance on behalf of CinemaBlend for a roundtable with Ang Lee to talk about Gemini Man and its new methods of pushing the visual envelope that Lee loves to poke and prod at any chance he gets. In that discussion, he explained why Junior had to be a CG character in the following, and greater detailed, answer:
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