Spider's scenes in Avatar 2 had to be filmed twice, two years apart
Per director James Cameron, Jack Champion filmed his scenes as the Na'vi's human adoptee twice: once as a reference for his co-stars, and once live action

Jack Champion as Spider in Avatar: The Way Of Water Image: 20th Century Studios
Since Avatar: The Way Of Water’s big release saw big success, director James Cameron hasn’t been shy about sharing details on how he did it, and what comes next.
In a new interview with Vanity Fair, Cameron highlights just how involved production was for The Way Of Water by sharing the “hideously difficult” process of filming scenes between Na’vi and humans. The crux of the complication: Spider (Jack Champion), a young human who grew up on Pandora and learned to live the Na’vi way (despite his notably smaller stature and inability to breathe their air without a mask.)