It was around 2015, some six years after “Avatar” was released, that director James Cameron reached out to the film’s leads, Sam Worthington and Zoe Saldana, to talk them through a potential sequel (or four). “We saw kind of a design element of the story,” recalls Worthington, who returns as human-turned-Na’vi Jake in the latest film. “It was like a campfire tale done in beautiful pictures right up until potentially the end of the saga.”

Saldana, who plays Na’vi warrior (and Jake’s partner) Neytiri, interjects: “That’s when he [Cameron] told us it’s gonna be in the water.”

Perhaps it was inevitable that Cameron, who as well as being a director is also a deep sea explorer, would eventually incorporate two of what appear to be his greatest loves: the Avatar universe, to which he has dedicated the last two decades of his life (and plans to decade at least one more), and the ocean.

The result, “Avatar: The Way of Water,” sees Cameron again push the boundaries of art and technology by adopting motion capture technology underwater, a feat that had never been achieved before. For the actors it meant a new challenge: not only spending a lot of time in the water (including in the real ocean on a research trip to Hawaii, where they underwent dive training) but learning to act in it.

Variety sat down with the film’s cast, including Kate Winslet and Sigourney Weaver, to find out what underwater acting involves and, at the peak of their training, who could hold their breath for longest below the surface.

Is acting in water different to acting on land? And what are those differences?

Kate Winslet [plays Ronal, a leader of the ocean-dwelling Metkayina clan]: It’s really different because you have to work so hard to tune out all of the other things. So there’s a countdown to when you’re taking your big peak inhalation before you go under. Once you get down there, everyone has to do a certain sequence of signals to let everyone else know that they’re okay. There’s a lot of technical stuff that you have to go through before you can just perform and it is quite strange. Obviously, everything happens much slower in the water, your body doesn’t always move as quickly as you might like it too. Especially if you’re doing a battle sequence. [It was] really frustrating realising that I wasn’t as good with the spear as I was on dry land, that was very irritating.

Sigourney Weaver [returns as Kiri, the adopted teenage daughter of Jake and Neytiri]: I think it’s just the same. You’re playing the character. I used to get into the pool early, like 15 to 20 minutes ahead of everybody else, because I needed to calm down because I’m a little claustrophobic and I wanted to get all the weights and the – there’s a lot of stuff before you even get to do the breathe up. So the more time I had, the more relaxed I was, which I knew I needed to be, because Kiri’s very good at the swimming.

Worthington: You’re doing action scenes, and you’re doing emotional connective scenes in water. They’re hard enough to do on dry land for most actors. You take away the oxygen, and you put yourself 30 feet deep, you have to figure out the process. And the technique is about calming your mind and calming your heart-rate so you don’t burn the oxygen, right? So the best way to do that I found is you forget that you’re underwater, which sounds kind of crazy. But as soon as you realise what environment you’re in, you’re going to start burning more oxygen just by thinking, so you have to kind of just connect with the other actor, trust in them and hopefully the rest of it disappears. And you can hear Jim’s voice as well underwater, directing you, as you’re doing the scene, as well as calming you down and saying ‘I can see you’re wigging out a bit just, it’s easy. You’ve got this, trust in your technique.’ But then he also talks about what he wants in the scene. The fight scene was very difficult and hard underwater. The hardest thing about fighting with Quaritch [played by Stephen Lang] underwater was there’s a lot of choke outs and you could literally take all the oxygen out of the other actor. So you deal with signals, you trust the safety team around you to keep a watchful eye.

Clockwise L-R: Kate Winslet, Cliff Curtis, Sam Worthington, Zoe Saldana on the set of “Avatar: The Way of Water” (courtesy ofof Twentieth Century/Disney)

Kate, was there any overlap with the water work that you did in “Titanic” at all?

Winslet: No, it was completely different. Because [in] “Titanic,” we have a small scuba sequence towards the very end when the when the boat goes down, and Jack and Rose are totally submerged, and they lose each other. We did have to do some scuba work for that. But nothing to do with breath-holding, nothing at all. No, we were in proper tanks for “Avatar [2],” deep tanks. “Titanic” we never had anything quite as deep. So no, it was totally different, actually.

Was it scary getting into those tanks?

Winslet: No, it was completely amazing. I absolutely loved it. When you’re working with real experts who know how to keep you safe and know how to teach you to maximise your fullest potential in a situation like that one, and will look after you, it was an amazing experience. It’s not something you can just try at home by yourself. Things can go horribly wrong if you don’t know what you’re doing. I love the training. I love the experience of all of it. It made me feel bionic. And that’s not something you expect to feel when you’ve had three kids and you’re faced with such a new physical challenge. So it was amazing.

Saldana: Water terrifies me. But you know what else terrifies me? Becoming paralyzed from fear, and not trying something that maybe I can master if I just apply myself and I’ve never shied away from a challenge that Jim [Cameron] has presented to me back in 2006. And I certainly wasn’t going to do it now. It’s going to take a whole lot more time, because I’m a little older, bones are just heavier now. But I got to learn something about myself that I never thought I would ever acquire. And it’s just this this level of confidence underwater that was just so beautiful. I also got to have like a glimpse of what it must be like for him to go on his expeditions and see what he was seeing and feel what he was feeling. And that’s kind of extraordinary.

Stephen Lang [plays Colonel Quaritch]: Well, I mean, [I was] concerned to the extent that you want to do the best you can always – I’ve always got concern. I always want to come up to the mark, whatever it may be, or something like that. But yeah, of course, doing the water work was challenging and difficult. But we were really led by a terrific team of professionals who were with us every step of the way, making sure that we were as safe as one can possibly be when one is doing a knife fight 25 feet below the surface. I enjoyed pretty much every moment of it, even when it’s painful and even when it’s frustrating, even when I just felt things were really going to hell, you know, still, I can’t think of any place I would rather be than there.

Trinity Bliss [plays Jake and Neytiri’s youngest child, Tuk]: It was really exciting, actually. We had such a great crew and coaches and Jim around us and also fellow cast mates around us that were supporting each other that and never felt like we were in danger of being killed!

Jack Champion [plays Jake and Neytiri’s adopted son Spider]: It’s not like we were in danger but I will say, as far as intimidation goes, I do remember the very first day, about to do tank work, and I was like in a mo-cap wetsuit, I was walking up [to the tank] and you walk up like, three flights of stairs to reach the top, and then there’s like this giant metal thing slammed down to create these waves that’s really loud because [it’s] slamming the water, and Jim’s like “And action!” and people are yelling “Do the scene” and like dive underwater and so to look at, especially on my first day, I was like, “Oh, shit, I’m about going to that machine.”

Behind the scenes on “Avatar: The Way of Water” (courtesy of Twentieth Century/Disney)

How did the first day go in the end?

Champion: It went fine. Because, you know, months of preparation, but it was definitely intimidating. But it was never dangerous.

What kind of training did you have?

Weaver: I did have swimming lessons when I was young and so I always considered myself a good swimmer. But this whole thing was, I think, somewhat intimidating to all of us, but also a very cool adventure, which is irresistible. We had a great teacher, this guy who teaches the Navy SEALs, Kirk Krack, and we had easily a year of training, building up our breath capacity and doing these exercises and classes. What I loved about seeing the movie last week was we really do look at home in the water. It is our element. And I don’t think that’s something you can act. We had too many responsibilities underneath the water. And we had gotten all that out of the way. So we could just be and, you know, when Jim gives you a challenge, he also makes sure you have all the tools you need to pull it off.

Bailey Bass [plays Winslet’s daughter Tsireya]: We had to do some water training in the audition process. So that included holding our breath for one minute, doing a dive 12 to 16 feet under and collecting rings and bringing them back up and going diving again, treading water, doing laps around a tiny pool, just to make sure that we were capable of swimming. And honestly, I was not a great swimmer so my mom took me to like the local swimming centre to do lessons prior to the audition and I’m so grateful for that, because otherwise, I genuinely don’t know what would have happened.

At your peak training, how long could you hold your breath?

Saldana: Back when we were doing the training, and we had to shoot and you also wanted to be super cool around everybody, five and almost six minutes. Now? 30 seconds.

Winslet: Seven minutes and 14 seconds

Weaver: Well, I did do a breath hold of six and a half minutes. But I’m not moving. I’m doing a static breath hold. And I probably had augmented oxygen.

Bliss: Three minutes, 30 seconds.

Champion: 30 seconds. No, I’m just kidding. Five minutes, 33 seconds.

Bass: My record is six minutes, 30 seconds on 50% oxygen.

Flatters: Five minutes 30-something.

Lang: I can do about 23 minutes

No you can’t!

Lang: No, I can’t. I can knock off four and a half minutes like that, no sweat at all.

These interviews have been edited and condensed for clarity and length.