Long before the invention of the digital motion capture suit that allows Sigourney Weaver to transform into Kiri, the sylph-like teenage Na’vi in Avatar: Fire and Ash, exploring the layers of a performance was a slower and much more physical process.

It would leave Weaver, now 76, to sit for hours with her reflected image in the make-up chair. “I am always disappointed when I look in the mirror because after all the hair and make-up, I really want to see someone else there. And when I look and see me, I’m like, ugh,” she says.

It is perhaps unsurprising to hear that kind of self-criticism from an actor of her calibre, but it does sit oddly with the knowledge that to the audience, Ellen Ripley in Alien, Katharine Parker in Working Girl and Weaver’s countless other screen avatars are revered as distinct, three-dimensional women.

“I remember looking in the mirror and I did get very close physically, I think, to Dian Fossey [in Gorillas in the Mist],” Weaver says. “The clothes were so wonderful, they changed my physicality. So I can watch it on screen, but when I look in the mirror, and I’m doing the make-up, I’m always like, darn.”

Weaver then confesses to an even deeper disconnect: “I find it very difficult to realise that I have done all these different things. I can’t quite grasp it,” she says. “I never can remember I’ve done this before. Every time I start a job, I’m terrified. I go, oh my God, I don’t ... and then when I get to the floor, I hit the ground running. Something else takes over.”

In Fire and Ash, James Cameron’s $US400-million-plus, 197-minute third Avatar chapter, Weaver did hit the ground running – literally – wearing a combination of technologies that bring the film’s effects to life in real time, using a virtual camera that allowed the director to see a version of the near-finished effect via a monitor on set.

“I could only play Kiri by rooting around into my most early memories of that age,” Weaver says. “I didn’t want to do an imitation of a teenager, I felt like to fulfil this challenge from Jim – luckily he kept telling me how immature I was anyway – I did have to sort of dig deeper.

“Like many people, I had a very awkward adolescence – I was this tall when I was 11, my body was all over the place, and I was always being told by teachers that I was clumsy. I just remember it in a kind of excruciating way, that sense of trying to survive your own adolescence.

“It was really a privilege to have a good reason to go back and crawl back into that skin of that not-very-happy girl, and see where I was,” Weaver adds. “And my sense memories of that time, for me, were so exquisite in a way, so real, that I was able to find her physically.”

In a sense, that ridiculously expensive high-tech space suit, with its motion-capture dots, opened a channel between Cameron’s billion-dollar motion picture franchise and the kind of experimental, artistically lawless, low-budget stagecraft that Weaver was learning as a young actress in the 1970s in New York.

“First of all, philosophically, playing the strange creatures ... I played multiple schizophrenics, and girls with hedgehogs in their vaginas [in a New York stage sex farce titled Titanic in 1976, written by Christopher Durang and directed by Peter Mark Shifter].

“In the world of the 76-year-old, being able to play this 14-year-old who wants to live in trees, there’s such a connection spirit-wise,” Weaver says. “You have to be fearless and not worry about the result. In an off-Broadway production, you’re like a pioneer. It’s the same thing in this.

“What’s so remarkable to me is that once you accept the helmet and this suit, it’s really like a rocket ship to anywhere,” Weaver adds. “I get out on this empty stage with nothing on it, it’s just me and Sam Worthington, in his suit, and what’s so powerful is that Jim Cameron has taken away all these obstacles of hair, make-up, costume, camera angles and positions.

“It felt like I was at an early theatre rehearsal off-Broadway, and Jim will capture this magic and put it in the bottle for us, and we don’t have to worry about that,” Weaver says. “All we need to do is surrender utterly to the moment, and explore for real what the characters are trying to do, what they need to say, what they’re feeling.

“It’s the greatest luxury really to work in this [medium] as an actor, and I think it’s sadly much misunderstood by other actors who still think that we’re voicing these cartoons,” Weaver adds. “It couldn’t be further from the truth – it’s the most authentic work I think you can do.”

Weaver – born Susan Alexandra; she borrowed the name Sigourney from The Great Gatsby in her teens and kept it professionally – was raised in a TV industry family in New York. Her father, Pat, was an influential figure in the early life of the US network NBC, creating, among other things, the Today show. Her educational life reads like its own Gatsby-esque fantasy: the Brearley School, the Chapin School, Sarah Lawrence College, Stanford University and finally Yale.

As an actress trying to find her stage, she struggled to work out where she could fit in. And then one day she answered an open call in the New York theatre newspaper Backstage, and was introduced to Sir John Gielgud. The play was The Constant Wife, directed by Gielgud, and starring Ingrid Bergman.

“Because I was so discouraged at drama school from even having a career, going up and meeting someone like Sir John, first of all, seemed like a total fluke. What was I doing there? I was like an impostor,” Weaver says. “But it was actually perfect. The person who gave me my first job was Sir John Gielgud and that was very healing for me.

“I was nervous, but I didn’t care,” she adds. “I’d been told the worst, that I had no talent. So I had a kind of attitude toward auditioning, which was not exactly f--- you, but I know what I want to do with this, and maybe you’ll like it, and maybe you won’t, and that’s fine. I was the understudy, and the assistant stage manager, but it was perfect for me because I got to do all the rehearsals with Sir John.”

The rest, as they say, is motion picture history. She had a small part in Woody Allen’s Annie Hall (1977). Ridley Scott cast her as Ellen Ripley in the chilling horror sci-fi hit Alien (1979), returning for the more action-oriented sequel Aliens (1986), directed by James Cameron.

She played British embassy officer Jill Bryant in The Year of Living Dangerously (1982), opposite Mel Gibson and directed by Peter Weir. She was Dana Barrett in Ghostbusters (1984) and Ghostbusters II (1989). She was primatologist Dian Fossey in Gorillas in the Mist (1988) and Katharine Parker, the villainous boss in the workplace-call-to-arms comedy-drama Working Girl (1988).

Such a trajectory has put her into collaborations with a who’s who of the directing world: Allen, Scott, Weir, Mike Nichols, Ivan Reitman, Ang Lee and, of course, Cameron. What makes a director great is sometimes difficult to discern, and Weaver says “there’s probably a different answer for each of those very gifted directors”.

“What I will say is that I felt that I was so odd, that it took a very odd director to think of me often,” Weaver says. “I was six feet tall, and Peter Weir thought of me for this love story; Mel is shorter than I am and didn’t care at all. But I’m just very lucky that I feel like the good directors are all pretty unconventional, and I prefer that kind of project.

“You go on a journey with them ... the stubbornness they need to bring this film into being, they have to be the champion of that story,” Weaver adds. “You have to feel that kind of exclusivity about that director and that material. And when you feel that, then you realise you’re in great hands.

“I definitely feel that with Jim Cameron; no one else would even want to do these stories, let alone be able to do them,” Weaver says. “They’re impossible. You read the script, you’re like, well, how are you going to do that? And that’s not what he sees at all. He sees where he’s going, and somehow he has come up with the technology to make these stories possible.”

Perhaps the most significant thing about Weaver is that across the three decades I have been writing about film and television, in the many conversations with leading women about their early influences in cinema – either in the form of strong women of purpose, or more physical women of action – Weaver’s name inevitably comes up.

“That’s a lovely thing to hear,” Weaver says. “I’m very flattered that these brilliant young actresses would put me in any kind of position. I just spent the weekend doing a Comic-Con with 200,000 fans and the thing that really touched me, and frankly surprised me, was these women saying that watching me as Ellen Ripley made them understand they could also be strong women.

“There was something in the character – and I give [producers] Walter Hill and David Giler complete credit as well as Ridley – that transcended that moment and became, I think, a kind of every-woman character so that girls can see that you have to keep going, that no one might come and help you, that you can do it yourself, that you’ve got this.

“That was never my goal, of course,” Weaver adds. “I just wanted to play the character, but it’s very touching to me that [other actors] would say that that made a difference to them. And there are plenty of actors I watch, whose work inspired me and made a difference to me.”

For Weaver, whose work has taken her around the world and created a platform for her environmentalism and activism, the Avatar franchise is perhaps the first project that has connected the two worlds. Even in the realm of big-budget science fiction blockbusters, art can be political.

“It’s an interesting question; it’s almost too big for me to answer, but what I will say is that about a month ago, I was part of the ratification signing of the High Seas Treaty, the goal of which is to protect 30 per cent of the ocean by 2030,” Weaver says.

The event put her in the same room as Chilean President Gabriel Boric, French President Emmanuel Macron and representatives of the Pacific nations, Africa, the EU and Scandinavia.

“Maybe, and until this second I didn’t really think about it, but maybe the Avatar films helped us around the world realise that we’re earthlings, and we have a planet to protect,” she says. “Maybe it helped us go a little faster towards something like the High Seas Treaty and know instinctively that this is a huge milestone for us as human beings and a huge milestone potentially for the oceans.

“I do think it has hit us on a very visceral level. I think that it has touched people and maybe accelerated our growth into an entity that can perhaps, like the EU, but in a planetary way, work to make the impossible possible. We can get a little closer. That would be a wonderful thing. And I think underneath it all, we as artists are hoping we can have an impact like that.”

Avatar: Fire and Ash opens on December 18.

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