"I felt I'd exhausted the treasury and it was time to go back to work," Cameron says. "Avatar is a very ambitious sci-fi movie." The director's enthusiasm is evident in his voice. "It's a futuristic tale set on a planet 200 years hence. It's an old-fashioned jungle adventure with an environmental conscience. It aspires to a mythic level of storytelling."
Avatar is not entirely a new venture; Cameron wrote the screenplay 11 years ago, and it has featured on Empire magazine's list of the 12 greatest unproduced scripts in Hollywood.
"I was never bored of making features," the director says. "This has been a dream project of mine for more than a decade, but when I first wrote it, the technology was not advanced enough. So I stuck the script in the drawer until the technology caught up."
Now it has. "The film requires me to create an entirely new alien culture and language, and for that I want 'photo-real' CGI characters. Sophisticated enough 'performance-capture' animation technology is only coming on stream now. I've spent the last 14 months doing performance-capture work - the actor performs the character and then we animate it.
"We've set up a studio, and last week [Lord of the Rings director] Peter Jackson and Steven Spielberg were here trying out the technology. I said to them, 'Take my tools and play with them for a week.' They were grinning from ear to ear. It's a really exciting time because so many new things are now possible."
For all that, Cameron stresses that movies should ultimately be about the story. "Film-making is not about sprockets. It's about ideas, it's about images, it's about imagination, and it's about storytelling."
"I'm driven by curiosity. I want to know how everything works, from the Big Bang onwards. There are still huge areas of curiosity to fulfil. When you've got a great story, whether it's a feature or a documentary, you've simply got to pursue it."
Having not directed a feature film for so long, does the film-maker feel any added pressure? "No. There is always pressure to perform from one feature to the next. There are always high expectations.
"I remember going with a great sense of anticipation to each new Stanley Kubrick film and thinking, 'Can he pull it off and amaze me again?' And he always did. The lesson I learnt from Kubrick was, 'Never do the same thing twice.' Avatar is not like anything else I've done - nor were Titanic or Terminator or Aliens.
"I always want to find something mentally engaging. I'll spend many months completing the special effects on Avatar, and it will not be released until the summer of 2009. It's quite a challenge - and for that reason, I embrace it."
'The Exodus Decoded' is on Saturday at 9pm on Discovery Channel
