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That their first movie — as fruitful as the final product was — wasn't the smoothest experience may have been partly due to Cameron's unconventional courtship of Weaver. When she wavered on returning as Ripley in the sequel, Cameron approached Arnold Schwarzenegger's agent, who also represented Weaver, with the idea of Schwarzenegger taking over the film series. It was a way, once word filtered back to her, to coax Weaver into signing on. The gambit worked.

“The first few weeks on ‘Aliens’ was a bit rocky while we tested each other,� Cameron says. “After that, we’ve been fast friends forever.�

That long-running friendship and collaboration reaches an unlikely pinnacle in “Avatar: The Way of the Water," . Though Weaver co-starred in Cameron's original 2009 “Avatar,� her character, Dr. Grace Augustine was shot and killed by the end of the film. In 2010, while Cameron was sketching out what would eventually become plans for four more “Avatar� films, he met with Weaver to suggest a novel idea of how she might be reborn on Pandora.

Weaver would come back in an entirely new role: Kiri, the 14-year-old biological daughter of Grace's avatar. In the film, which opens in theaters Thursday, Weaver plays the Na'vi character, adopted by Jake Sully (Sam Worthington) and Neytiri (Zoe Saldaña). It's not easy logic to parse out. What, exactly, are the reproductive capacities of avatars?

“I had to jump through a few fiery hoops to get her on this movie. Not that she was unwilling, but in terms of narrative logic,� says Cameron. “OK, she’s the daughter of the avatar of Grace Augustine, you know what I mean? So of course she looks like Grace. Makes sense, right? Oh, by the way, you’ve got to be 14 years old.�

But the end result of Cameron's narrative logic is simple enough: Sigourney Weaver is back in the “Avatar� fold. And not only that, through performance capture she's playing a wide-eyed teenage Na'vi who looks a little like an alien Winona Ryder. As Weaver likes to say, in science fiction, “Everything is possible.�

“I know that no one but Jim would have cast me as a 14-year-old. As he said, people think I’m kind of serious and strong and all those things. He knows that that’s just malarkey,� says Weaver, smiling. “He said, ‘This will be easy for you.’"

It required a bit more work than Cameron made out. Like the rest of the cast, to film the movie's extensive underwater scenes, Weaver trained for the underwater performance capture work by learning how to hold her breath underwater for six minutes. “Six and a half,� Weaver corrects. Getting back into the headspace of a teenager also demanded some immersion. She spent time in high schools, she says. “I needed time to unearth my 14 year old.�

“I was this tall when I was 11, which was excruciating,� says the 5-foot-10 Weaver, the daughter of pioneering TV executive Pat Weaver and the British actress Elizabeth Inglis. “I still, at 14, spent half my time wanting to disappear. I think Kiri has some issues that are difficult for her. She’s the adoptive daughter. She’s very much part of the family, but she also has these things going on that she doesn’t quite understand.�

It's not a one-off performance, either. Cameron envisions big things for Kiri in future “Avatar� installments. He's already filmed the third film, begun shooting the fourth and developed the fifth. Fans of Weaver from “Ghostbusters,� “Working Girl� and “Galaxy Quest� will get one more chance to grow older, again, with the 73-year-old actress.

“She’s a very, very important character,� Cameron says. “If we’re lucky enough, in terms of enough financial success on the movie to break even and make it a proper business case going forward and we get to do 3'’ — we’ve already captured everything with Sigourney for ‘3’ — and then ‘4’ and ‘5.’ Her character has increasing importance across the greater saga. Both of us are looking forward to that exploration.�

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