James Cameron's epic "Avatar" has been a long time coming.

Cameron, 55, developed the idea for the film even before he made "Titanic" -- but technical difficulties got in the way of his vision.

"At the time it turned out that it wasn't technically possible to do what I wanted to do," he tells "Extra."

The film is about humans invading an alien planet called Avatar, where the Navi population lives.

"Avatar" stars Sam Worthington, Zoe Saldana and Sigourney Weaver. The film arrives in theatres December 18.