[Pippin]

I kinda laugh at the thought of Elijah Woods and Sean Astin having to interact with someone who's not there when they were filming scenes with the CGI.

[sweetliberty]

In the appendices to Fellowship of the Ring they go into great detail about how they created a lot of the effects and how they adapted the characters to various scenes. They are apparently planning to do similar appendices to The Two Towers when it is released. I expect that much of the focus will be on gollum and how the other characters learned to relate to him.