[b3orn]

But that only works if you already know the exact movement and you basically end up playing a film on the background screen walls. You can't change anything of this on set anymore. Using real-time rendering with Unreal or whatever gives you more flexibility in exchange for visual quality.

[RichardCA]

It's done that way because of pre-visualization or Previs, which has been the norm for over a decade now.

https://www.youtube.com/watch?v=oxTNhNe6Fbc