Interviews with American actress Sigourney Weaver tend to begin by marvelling at how tall she is, so here goes. Actually, while Weaver is tall (an elegant 5f t 11in), going by some of her old interviews it occasionally seems overplayed - almost as if some giantess had stalked into the room, growling 'Fe fi fo fum' and gobbling up the tape recorder. According to Weaver, 'the height thing' has aff ected her entire career, particularly where casting is concerned. 'Producers are short, I'm tall - I'm not the average producer's sexual fantasy.'

'It's true,' says Weaver, as we sit and chat in a central London hotel. 'A lot of actors are short and I've walked into rooms and had people go ...' (she lurches back in her seat, pulling an 'astounded' face). She laughs. 'But I think it's been a blessing. Most people didn't know what to do with me, so I ended up working with Ridley Scott and James Cameron, people who didn't much care for the conventional stuff .'

Weaver, not so freakishly tall after all, but still in possession of that angular, slightly patrician beauty at 56, proves to be warm, intelligent company, maybe slightly more quirky than you'd imagine. Beautifully groomed, with a thick brunette bob and mischievous eyes, she comes over as a cross between a 'movie lady president' and the retired 'hippy chick' she actually is. (Readers of a sensitive disposition should be warned of references to 'flute playing' and 'elf outfits' appearing later in this piece).

Accompanying Weaver for this interview is Ros, an autistic British woman whose behaviour she researched to play a bereaved autistic mother in her latest film, the compelling, poetic Snow Cake. Weaver says she was thrilled to be considered for such an unusual role. 'Usually all Hollywood wants you to do is what you just did,' she says. After The Ice Storm I was offered a thousand Ice Storms and so on. You always get offered the same thing again and again, if you're not very careful. It's up to you to swing back and forth.'

Born in New York in 1949, Weaver had a privileged upbringing as the daughter of English actress Elizabeth Inglis and Sylvester 'Pat' Weaver, president of the NBC network in the Fifties, pioneer of The Today Show and The Tonight Show. Originally christened Susan, she changed her name at 14 to Sigourney after a minor character in The Great Gatsby. Educated at Stanford and then Yale drama school, the way Weaver describes her youthful self sounds almost goofy - a far cry from the elegant, slightly austere public persona she has today. 'I was silly, funny, always the clown,' she says. 'One of those people who take so long to coalesce into a definite person - just this big blob going through adolescence, trying to take in everything.'

At Stanford, Weaver spent a bizarre period living in a tree-house with a boyfriend, playing flute duets and wearing homemade elf outfits. Explanation please? 'It was the Seventies, you know - flower power!' Wasn't she a debutante as well? 'Well, yes,' Weaver chuckles dryly. 'My mother being English always felt that if Prince Charles were to fall in love with me I'd have to be a debutante or something.'

Surprisingly, given her propensity for elf-dom, Weaver rather enjoyed 'coming out'. 'I was so tall and gawky and shy, that, for me, getting the dress, having my hair put up, was a great adventure. I was like - "Oh my god, look at me, I'm a girl, I'm pretty!"'

Weaver felt miserable and undermined at Yale (where Meryl Streep was a contemporary). 'It was terrible, this big blanket of rejection'. She contemplated giving up acting. Things started coming together for her while working in New York in theatre, where she still has firm roots. Her husband, Jim Simpson, runs The Flea, an acclaimed New York theatre. They have a daughter, Charlotte, 16. For about five years Weaver ran her own production company, Goat Cay. 'I was trying to get money into the pockets of playwrights, get their voices and innovations into film. But once independent film came in that happened more naturally anyway.'

One project Weaver did with Simpson was The Guys, a play dealing with the immediate aftermath of 9/11, which was eventually turned into a film. 'We were in New York on 9/11, and it was like suddenly downtown Manhattan was frozen, paralysed. So the young people my husband worked with said we had to use the theatre to come to terms with this terrible tragedy.'

In the end, The Guys ran in the theatre for a year. 'Our President said, "Move on! Spend money! Don't look back!" Which was crazy - New Yorkers had nowhere to go to experience their grief and loss.'

Was Weaver anti-war? 'Yes, for all the good it did, but now we're there...' She shrugs, defeated. 'Oh, I don't know.'

What does Weaver make of the Bush-Blair axis? 'Well, we always like to see Blair because he can actually speak in a complete sentence. You're not spending the whole time terrified of what he's going to say. Every time there's a crisis I think, "Why did this have to happen now, why can't we wait until we have another President?" We're just counting the seconds until we can get someone else in there, even if he's a Republican. It's so frightening to have a man like that be our President.'

Weaver sighs: 'I just find the administration very arrogant but I'm not sure I'm comfortable criticising my country when I'm over here, so ...' She lets her voice trails away.

After her spell in theatre Weaver made her film debut in Annie Hall (on screen for six minutes), and, at 29, was offered the career-transforming role in 1979's Alien. 'I didn't want to do it. It was sci-fi, and I was a total snob - I wanted to do Mike Nichols, Woody Allen, Shakespeare, and that's about it.'

That's interesting, I tell her. The way she portrayed Ripley could be considered rather Shakespearean.

'Well, I decided, in this case, I was playing "Henry V on Mars",' says Weaver. 'That was how I transformed it into something that was compelling for me.'

You wonder whether it irks Weaver that, after such a lengthy varied career, she is still to a large degree defined by Lt Ellen Ripley. She has appeared in films as diverse as The Ice Storm, The Year of Living Dangerously, Copy Cat and Gorillas in the Mist (for which she was Oscar-nominated for Best Actress in 1988). She has also more than proved her 'comedy chops' in vehicles such as the hugely likeable Ghostbusters. Weaver tells me she is dismayed that her career has not featured more comedy: 'It is the thing I love to do more than anything else.'

Now there is Snow Cake, co-starring Alan Rickman and Carrie-Ann Moss. Weaver portrays Linda, an autistic woman whom Rickman befriends in the Canadian outback after her daughter is killed in a tragic accident. Being an evil journalist, when I first heard about Ros's presence at the interview I briefly wondered whether this was actorly 'showing off ' about how far people go in the name of research, or simply a great diversionary tactic. A few minutes into the conversation I realise my cynicism is ill-placed. Ros is unusual for an autistic person because she has trained herself to respond to people. (Just before the interview I sidled up to her and apologised in advance if some of my questions left her out. 'You can't off end an autistic person,' she said nicely but brusquely). Weaver simply seems at pains to point out what a great help Ros proved to her in developing 'Linda'.

'I'd been doing my own research but the spectrum of autism is so huge,' says Weaver. 'I can't imagine what I would have done if Ros hadn't coached me - the walk, the playing, everything really. I just felt this huge responsibility to get things right.'

There have not been that many movies dealing with autism - did the spectre of Rain Man loom large? 'Rain Man was like a million years ago and nothing else has come out since then,' says Weaver. 'You can't have this very brilliant specific interpretation represent everybody with autism for ever.' She laughs. 'Enough already!'

Later, when I speak to him on the phone, Snow Cake's director Marc Evans praises Weaver's performance for managing to portray a character rather than a condition: 'You know the moment Sigourney Weaver enters the frame there's going to be this expectation or judgment - "Come on Sigourney, give us your autism" - but it isn't like that at all. There is something about Sigourney's own strength as a person and an actress that manages to come through into the character.'

Despite Weaver's versatility it is still her turn as the sweaty, determined Ripley, fighting off space terrors in a grimy singlet, that remains burnt into the public imagination, sometimes to an amusingly 'pervy' degree (I looked on the internet and Ripley is a huge sci-fi sex symbol. Certainly the Alien franchise has been good for Weaver. She received an Oscar nomination for the sequel to Alien, Aliens. For Alien: Resurrection she was paid £ 6m and given a co-producing credit, making her a serious Hollywood player.

'I totally credit Alien with giving me weight in the industry,' says Weaver. Does she think it's interesting that two of the strongest, most iconic feminist screen characters (Ripley, and Linda Hamilton in Terminator) both came from the sci-fi genre? 'As opposed to the real world?' says Weaver, raising an eyebrow. 'What was considered daring at the time was that we had this ordinary working woman, not in a frilly space dress, but a woman with dirty hands, dealing with people in a crisis. And it's still something they don't do. It's as if they feel they have to turn the woman into some kind of weird science-fiction doll to make it interesting.'

Of her career generally Weaver says: 'I've always been drawn to women who aren't very comfortable in the world, who are quite isolated because of their great passion for something. For me, because I was such an awkward teenager, I just love sending out the message that women can come in all different shapes, sizes and feelings, and it's all normal. It's normal to be abnormal.'

Now in her late fifties, Weaver is clearly over the 'height thing' and refuses to subscribe to Hollywood's uniquely paranoid brand of the beauty myth. 'I don't think it's attractive to have a taut face with a 65-year-old's body. I find that look scary.'

She also finds the modern emphasis on celebrity bemusing: 'I just don't understand the great urgency to find out that we all eat breakfast and go to the john. I mean, what's the big deal?'. However, ask Weaver if she feels that Hollywood is a cruel industry for women, and she'll answer that it's hard on everybody (a kind of equal-opportunities cruelty). Towards the end of our interview she relates what happened the night of the 1988 Oscar ceremony where she was nominated for both Gorillas in the Mist (for best actress) and Working Girl (for best supporting actress) but ended up winning neither.

'It was the first time in history somebody had lost twice in one night,' Weaver chuckles. 'And when you lose in Hollywood, everyone goes [she huddles behind her hand, looking aghast], "She lost, she's a loser". I felt like there should be a "Loser's Tent" for all of us to go in together and smoke and drink and swear because people look at you with such incredible pity. You just want to say, "I'm fine, I'm exactly the same person I was before I arrived at this show."'

And what sort of person is that, what sort of actress? Weaver smiles: 'My early role models were people like Jessica Tandy, and they were always working - they would do film, they would do theatre, they would take anything that came their way so long as it was decent. And I love that attitude. I don't want to sit in an ivory tower, waiting for my big movie to come up once every five years. That's so boring.' Weaver waves her hand dismissively. 'I want to be out there, working with different generations. That's what's always been so fun about it.'

· Snow Cake opens on 8 September. Sigourney Weaver also stars in Infamous, about the life of Truman Capote, due to open this autumn

Sigourney in short

Born Susan Alexandra Weaver in New York, 8 October 1949. She changed her name aged 14 to Sigourney, after a character in The Great Gatsby

Family Father, Sylvester 'Pat' Weaver, was president of NBC and mother, Elizabeth Inglis, was an actress. Sigourney married theatre director Jim Simpson in 1984. They have a six-year-old daughter, Charlotte, pictured right as a baby.

Studied English at Stanford and Drama at Yale.

Breakthrough Weaver appeared for six seconds in Annie Hall (1977), but it was as Ripley in Alien two years later that she made her name.

Career highlight In 1989 Weaver won two Golden Globes for her performances in Gorillas in the Mist and Working Girl

She says 'The movie business divides women into ice queens and sluts, and there have been times I wanted to be a slut more than anything.'

Ripley & co

Alien (1979)

Ellen Ripley, Weaver's best-known and ballsiest character, was introduced in this first of four sci-fi thrillers.

Gorillas in the Mist (1988)

Weaver played ill-fated gorilla activist Dian Fossey in a real-life story.

Working Girl (1988)

Played opposite Melanie Griffith as her idea-thieving Wall Street boss.

The Ice Storm (1997)

Excelled in Ang Lee's film about two troubled families in Connecticut.