Some of it's Tolkien and some of it's Jackson's strengths as a director. He shows us ordinary people - the families in Rohan, the people in Gondor watching the doomed soldiers leave, even a few orcs - with enough there to make the extras mean something. The Na'vi extras, and the human extras for that matter, were just there.

And even Grima wasn't just doing things for the Evuls! and neither was Gollum. Their motivations were twisted, Grima's mostly by ambition and whatever name you'd give his feelings for Eowyn, Gollum's by the ring, but you knew they had those motivations.

Cameron can do better, too! Look at The Abyss. I think he should do himself and his storytelling a favor and stop pushing for bigger and flashier.