Lots of scenes are cut/edited in. The actor isn’t really looking at the “scene” as the viewer is. CGI is everywhere.
The set is usually busy and brightly lit. What the viewer sees is immersive – how the director wants it. But the set itself has people walking around, moving lights and cameras etc. This is why they’re called actors – they’re faking it. It is far less scary when you see a person manipulating the controls of a robotic dummy getting killed, for example.
Latest Answers