Director James Cameron is writing an Avatar "bible" as he prepares to start working on a sequel to his movie blockbuster. 

The filmmaker has previously revealed he's planning to set the follow-up to the 2009 sci-fi epic underwater, but the film's star, Sam Worthington, warns fans not to expect anything from Cameron for a while - because he's still trying to get back into the Avatar frame of mind. 

The Aussie actor says, "Jim told me what it is, what he's got planned, it's huge. 

"The thing with Jim is he doesn't want to disappoint anybody and he won't be popping out a sequel just because he can, so he's actually taking his time writing a big bible, a pre-cursor to Avatar One, just to get himself back into that world." 

But Worthington is confident the wait will be worth it. He adds, "It's definitely huge."