It’s hard to believe, sitting opposite a poised and self-possessed Sigourney Weaver, that she was once a painfully insecure, awkward little girl desperate for someone to tell her she was pretty.

“I had an English mother. She was always trying to make sure that I wouldn’t be conceited, to the point where I had a real inferiority complex. I would say, ‘Gosh, Mummy, am I pretty?’ And she would say, ‘Well no, you’re plain, dear, but never mind.’ I think it’s an English thing. There’s this feeling that you’re going to spoil your child if you keep saying how wonderful they are. I thought, gosh, if my mother doesn’t think I’m pretty, I had better crawl under the couch.” She’s laughing as she recalls her gauche younger self, so clearly no permanent psychological damage was done.

At 58, Weaver is, to use fashion parlance, beyond fierce. Blessed with a sculpted, patrician bone structure, velvet brown eyes and the lean physique that wears clothes like a compliment, she looks effortlessly chic in a tangerine lace skirt, a cashmere top and funkily mismatched jewellery. In a matter of seconds, her conversation can veer from an earnest observation on the state of the planet to a fantasy riff about taking a robot buddy to the Hampton Court Palace Flower Show (“Robot, what do you think of these perennials?”).

Weaver is in town to promote the latest Pixar animation, WALL-E, a charming movie about a lonely little robot left to clean up planet Earth. She’s as much a fan of the film as a participant. Her role is small but Weaver is canny enough to realise that her presence on the press tour can only help the film’s prospects. “I don’t have to be on tour for WALL-E,” she confides. “I feel a little bit like an ambassador for a movie I just admire so much. It’s a film I want all earthlings to see; it brings us together.”

It’s impossible not to be a little star-struck in the presence of Sigourney Weaver. This is the woman whose empathetic portrayal of Dian Fossey earned her an Oscar nomination for Best Actress the same year as her turn as the treacherous Katherine Parker in Working Girl secured her the nomination for Supporting Actress. She brought sympathy to the role of the self-destructive suburban wife Janey in Ang Lee’s The Ice Storm, and damaged rage to the torture victim Paulina Escobar in Polanski’s searing Death and the Maiden. Most iconic of all, however, is the character that launched her career, Ellen Ripley in the Alien series. As Ripley, Weaver was the first true female action movie star. She redefined the role of women in the sci-fi genre – before Ripley, that role seemed to comprise mainly of wearing a metal mini-dress and futuristic eye make-up.

But Weaver has never taken herself so seriously that she couldn’t send up her status as a sci-fi legend. In Galaxy Quest, she played a cast member from a cancelled TV show whose job on the space-craft was to repeat everything the ship’s computer said. And now in WALL-E, her casting is a sci-fi buff’s in-joke: as Ripley, she did battle with the autocratic ship’s computer, Mother; now Weaver is the voice of the computer of the Axiom, the spaceship into which humanity has been exiled until a polluted planet Earth is habitable again. We all turn into our Mother, one way or another.

Weaver insists that there is common ground between playing, say, the butt-kicking, alien-grappling Ripley and a high-functioning autistic woman in Snow Cake. “I’m just a completely story-based actor, if you look at my choices. They are all over the place genre-wise, but I always try to tell a story with a beginning, a middle and an end. I’m old-fashioned like that. One of the reasons I admire Pixar so much is that the story is always the king. More and more in our business, there are elements that are important to studios, but the story is not always one of them, unfortunately.”

Another attraction of the WALL-E script was the environmental message, which touched a nerve with the former flower-child. Bizarre as it might sound, Weaver was once a hippy who lived in a treehouse, played the flute and wore an elf costume. It’s a period she shrugs off as part of a learning process.

That process started at the age of 14 when Weaver, christened Susan, took the name Sigourney (after a minor character in The Great Gatsby). The reason, she explains, was that, as a tall teenager, the diminutive versions of her name felt inappropriate. “Sue sounds like a little person’s name. So I thought, here’s an interesting name. It’s long, and I’m long. It’s unusual. So I’ll just use that for a while until I can figure out what name to use. It was like a temporary thing. But of course, now everyone calls me Sig or Siggy, which is just like Sue or Susie.”

So what advice would she give her 14-year-old self if she could go back and talk to her? “I would say, it’s going to be OK. I was always worried whether things were going to work out. There is no perfect path. In fact, it’s the odd detours that make life interesting. Try and relax and enjoy the fact that your road is going to be different to other people’s.” A PR woman pokes her head around the door to check how we’re doing. Weaver chides her: “You can’t stop us, we’re just getting to the most interesting bit!”

Weaver’s road continues to take in interesting projects. Out next is Baby Mama, a comedy in which she stars with Tina Fey and Amy Poehler. And Weaver has just finished work on James Cameron’s top-secret sci-fi project Avatar. She can’t reveal anything about the story but does say that she thinks her character, a capable, nononsense scientist, is the closest thing in the film to Cameron’s alter ego. She says of the ever-in-creasing workload: “I do enjoy it so much, more than I ever have. It doesn’t feel difficult to me.”

But the ruthless, disposable film industry today is a different world from the one in which Weaver got her initial breaks, and that troubles her. The appetite for celebrity trivia – and celebrity failure – is something that has started to blight the film world in the past ten years. “I’m just trying to figure out why that has happened,” she muses. “I still remember the black and white films that you would watch and go, how did they do that? I think people used to sit in theatres and they could see the skill. Nowadays we tend to make it look easy and contemporary. I don’t know that people can see that actually we do have to have a gift and have experience. There’s this tendency to try and tear us down to see if we are that special. I think we need stars. I want to believe in movie stars; I don’t need to know what they had for breakfast. I’d like to believe that George Clooney is as wonderful as he seems.”

With all that in mind, how would she feel if her 18-year-old daughter Charlotte (with her husband, the theatre director Jon Simpson) announced that she wanted to follow her mother into the family business? “My daughter is always being cast in things, but she wants to be a costume designer. Who knows what will happen? Her relationship to the industry is on her terms. I think you only end up in this business if it’s the only thing that will make you happy. If that’s how she feels at the end of college then more power to her. I’ll totally support her. I think my daughter is one of those people who’ll have a good time whatever she chooses. She’s not a tortured soul; she’s a cork who’ll bob through life.”

WALL-E goes on general release tomorrow

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