Watch: Zoe Saldana and Sam Worthington talk Avatar: The Way of Water tech
The stars reprise their roles as Na'vi warrior Neytiri and Jake Sully — a human who now lives permanently in the form of his blue-hued avatar.
Director James Cameron's long-awaited sequel introduces the couple's children and tells the story of their decision to flee their homes due to the threat of the returning 'sky people'.
They find refuge with the Metkayina — a clan of Na'vi who spend much of their time underwater — which meant the actors had to learn to dive, and dive well.
"[Cameron] had been saying for years that the next challenge, and the next step, is to go into the water," Saldana told Yahoo UK.
"And when you hear him say that, you kinda go 'that's great', but then you're thinking 'what does he mean by that?'. You never really know what he means until you see it, until you feel it."
Worthington added: "The first time it was very unique and that's what made it ground-breaking, because no one had tried it before to the level that we were doing it.
"You kind of know that you're gonna be guinea pigs again with Jim. He's always gonna push it."
Worthington explained that everything they did for the original film in 2009 was now given an extra layer of complexity by the fact much of the movie was shot in water tanks.
He said: "The technology for us doesn't change, but the environment changed. Where we film, called the Volume, was now underwater.
"But we knew that was gonna lend itself to the authenticity of what we were doing. That's a new challenge, but that's what Jim is about.
"Jim isn't gonna make a movie if it's just the same story and the same tech. He's gonna keep pushing what film can do and what tools he's got."
Worthington said the new film has 'an immersive quality', especially in the gasp-inducing underwater scenes.
He added: "This is a movie that you're going to experience it, not just see it. As you said, you were gasping. I think that's what they were using.
"Jim has got a great set of tools now that he's a master of, and that's what he wanted to use to continue this saga."
Both actors faced a huge time commitment in choosing to return to the Avatar franchise — especially with many sequels to come — and conceded that they have had to say no to other projects.
Saldana said: "We can't wait to work with Jim. He's just such a collaborative and amazing partner, and an amazing leader to lead the battalion into battle.
"We get to relate to peers and colleagues that we truly respect and we're growing alongside them. That becomes an adventure."
Read more: John Oliver takes a swipe at Avatar sequels
And Worthington wasn't worried about the prospect of audiences simply not turning up, despite the 13-year gap between movies.
"The weird thing is that no one knew anything about the first movie and it didn't seem to hold that back," he said.
Worthington added: "You just hope that you've told a compelling story that has deep roots and deep emotions, and you hope it connects. That's all we can do.
"Jim is this masterful storyteller in the sense that he deeply layers his great ideas about what we're doing to the environment and ecosystems and, in this case, what we're doing to each other. He puts them into his movies and, if they connect, they connect."
The Avatar sequel also includes Sigourney Weaver and Stephen Lang among its returning cast, with new stars including Kate Winslet joining the fray.
Avatar: The Way of Water is available in UK cinemas from 16 December.
Watch: Trailer for Avatar: The Way of Water