That magic little piece of technology is called motion capture. Most definatly NOT something that Mr Cameron invented.
Additionally, while he may have been able to see the cgi characters in realtime, they would need to be realtime models with realtime capable effects/textures/lighting. Its not going to be of the quality the final product has. That kinda crap still takes renderfarms butt-tons of time to render out. His "invention" is only a tool to help him visuallize how the scene will look in the end. Note: I'm only refering to the tool James Cameron "invented".. not Motus