content: I saw a documentary on CGI once (it was a focus on Industrial Light and Magic, one of the biggest CGI houses) - some of the actors (Robin Williams and Samuel Jackson I think) said it was basically the directors describing the scene to them and telling where they should move, where they can't walk (because something CGI will be in the way), and it's up to them to imagine it. Sometimes a green dummy is put in place so the actors have a reference point, but sometimes they just have to use their own imagination - I remember they showed one scene in the Star Wars prequels where Yoda was moving on a levitating platform thing and Obi-Wan and Mace Windu were having a conversation with Yoda. There was no Yoda or any platform, the 2 actors just had to look down and pretend there was something there.