Titanic star Kate Winslet is set to appear in the much-anticipated Avatar sequel and director James Cameron has revealed certain details about her role in the sci-fi film and teased an underwater theme.

Cameron spoke to Vanity Fair about Winslet's character, and explained, "She's very excited about it. She blazed through for a couple of days of rehearsals and saw the world that we had created, and how we do the work, and she's very excited."

Revealing that the Oscar winner plays a "Sea People" character, he continued, "She plays a character who's part of the Sea People, the reef people. The one thing she did do is demand that she do all her own water work. I said, 'All right, that's fine, we'll have to teach you how to free dive.' The other actors are up to three- and four-minute breath holds."

As already confirmed, the movie partly take place underwater and the director shared some filming details about some submerged scenes and their sign language. "We've already been doing underwater capture. We did a scene last week with six teenagers, well, actually five teenagers and one 7-year-old underwater holding their breath for a couple minutes and acting, actually doing a dialogue scene under water because they speak kind of a sign language," he said.

The first Avatar movie came out in 2009, where paraplegic marine, Jack, was dispatched to the moon Pandora on a unique mission, but after living with the special tribe he ends up in a dilemma on where his loyalties lay. The movie ended with Jake permanently transformed into his Na'vi avatar as the rest of the earthlings, who attacked the local Na'vi tribes along with Colonel Miles Quaritch, were forced to retreat back to the earth.

Addressing the script delays, the director explained, "I wouldn't call them delays. It was highly optimistic that we could start quickly until scripts are written. If there's no script, there's nothing, right? The scripts took four years. You can call that a delay, but it's not really a delay because from the time we pushed the button to really go make the movies [until now,] we're clicking along perfectly."

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Jake and Neytiri in Avatar movie 20th Century Fox

"So when all the scripts were approved, everything was designed, every character, every creature, every setting," he added.

The Terminator director has also revealed that most of the actors have already read their scripts. "Most of the actors, the key principals, have all read all four scripts, so they know exactly what their character arcs are, they know where they're going, they know how to modulate their arc now across the first two films."

However, the 63-year-old award-winning director has warned that if Avatar 2 and 3 is not box office success, Avatar 4 and 5 may not be made. "We all know where we're supposed to be dramatically in the saga, and that's great. Let's face it, if Avatar 2 and 3 don't make enough money, there's not going to be a 4 and 5," Cameron quipped.

Teasing a "meta-narrative" he continued, "They're fully encapsulated stories in and of themselves. It builds across the five films to a greater kind of meta-narrative, but they're fully formed films in their own right, unlike, say, The Lord of the Rings trilogy, where you really just had to sort of go, 'Oh, shit, all right, well I guess I better come back next year.' Even though that all worked and everybody did."

The first Avatar sequel is slated to open on 18 December 2020.

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James Cameron and Kate Winslet at the premiere of Titanic 3D in 2012 Getty Images