[Scott from the Left Coast]

Fascinating. Thank you.

What does it mean to "shoot empty plates"???

[maquiladora]

I think what he means is that they shot the scene with the actor actually there with Frodo and Sam, playing the part and speaking the line, then they shot the exact same scene again, this time without the actor so that there was just nothing in his place, then they introduced the CGI gollum into this scene without the actor, and layered it onto the scene where the actors where acting/speaking to the real actor.
Think about it :-)