LONDON. "When the script arrived, I laughed," she grins. "I thought, Robin Hood? Come on! About forty-eight hours later, I was facing Kevin Costner over a ditch in deepest, dampest Northumberland." Replacing the pregnant actress Robin Wright (also Mrs. Sean Penn), Mary Elizabeth Mastrantonio became Costner's Maid Marian -and the heroine of a $50 million blockbuster.

But sprawled on the floor of a Notting Hill flat in beat-up jeans and black sweater, Mastrantonio looks more like a college student on vacation. She is fresh, pretty and petite, with a heart-shaped face free of makeup. Her denim jacket lies nearby, slung over a pink nylon backpack; her sneakers are scuffed and businesslike, hardly designer-label items. "This afternoon," she is telling me, "I have to make myself go shop for clothes. And shopping is the one thing in life I really detest."

In person, as onscreen, Mastrantonio projects a remarkable warmth and openness. But, it transpires, she does not really trust her own charm. Perhaps because, as she puts it, "Hollywood is run by men who are big on 'vulnerability'. Other women see all the women I play as separate people. Men say, 'She always plays tough, willful types; strong women who are also vulnerable'."

Mastrantonio smiles, and her whole face is transformed. You suddenly notice her perfect bone-structure, the startling luxuriance of her wavy hair. The smile captures her fascination with finding the fun in life - with taking time to make a joke, get a kick out of things, listen to others. In her profession, this talent is both well-known and much-appreciated. "Mary Elizabeth is very funny; very naughty, too," says Briton Constantine Gregory, an actor and dialect coach who worked with Mastrantonio on 1983's Mussolini, well as on Robin Hood. "She's one of those rare people who creates a great sense of fun and loyalty on a set."

Not that Mastrantonio takes her work less than seriously. Just that she sees it as only one part of her life. "I hate it when people say, 'Mary Elizabeth, this may be hell, but the movie is going to be sooo good'. Cause I'm not looking at money, percentage points or grosses. This is my life, you know? To me, every day matters."

For eighteen months now, Mastrantonio has worked from a base in Britain - a result of her marriage to Irish director Pat Connor. (The two met when Connor directed her in 1989's The January Man.) But only last week did the couple moved into the London "dream home" they purchased last year. And in two days, Mastrantonio must set off for New Orleans: to "raise Robin Hood consciousness" on a press junket around the US. The role she took "because it was filming in Britain, so I could come home at night" has taken her career into the big-leagues of bankability.

It's not a moment too soon. In terms of career vicissitudes, Mastrantonio has needed her sense of humour. Her film debut in Scarface, as Al Pacino's sister, proved that she could exercise a vivid screen personality. But the roles she subsequently landed were often in promising films (The Colour of Money with Paul Newman and Tom Cruise, The January Man with Kevin Kline, with Ed Harris) which, in the end, failed to deliver. "It got to the point where all interviewers ever asked me was, 'Al Pacino, Paul Newman, Tom Cruise! Tell me, what was it like?'"

Then came this year's Class Action, an old-fashioned courtroom drama in which Mastrantonio's yuppie lawyer opposes her own father (Gene Hackman). He is an '60s liberal but a failed family man; she is a '90s pragmatist who has much to learn about ethics and forgiveness. Action reminded critics just how good Mastrantonio is - especially in her emotional private scenes with Hackman.

"Gene is just so great," she says now. "There's no mystery about what he's doing, no voodoo, nothing hidden. He doesn't bully and he doesn't test you. He doesn't go off in a corner, and he doesn't flounder around."

Mastrantonio pauses a beat. "Some stars like to hide behind the whole idea of acting. But really good actors are not hiding at all. They're not afraid to be disliked, to be a little unsavoury. I've worked with leading men so worried about losing their charm that they were always winking to the audience. As a measure of acting skills, film can be very deceptive."

If she hadn't been cast in Scarface, Mastrantonio's leading men might have had names like 'Placido' or 'Luciano' instead of Gene or Al. Born outside Chicago, where her father worked in a foundry, Mastrantonio studied voice at the University of Illinois. She hoped for a career in opera. But a chance role in West Side Story took her to New York. And, from '79 through '82, she was onstage there every night in a string of musical-comedies. Many Manhattan theatre pros remember her from this period with fondness. Says the Obie-winning lighting designer Frances Aronson, "Mary Elizabeth was loved by the crews as well the other actors. Now, I hear it from staff on her films: 'The crew will do anything for her'."

In 1982, Mastrantonio landed a speaking role in the long-running Amadeus. "Suddenly, I realised: this was what I wanted to do. I didn't know how to do it; I just knew acting felt right." But she found an acting coach. And, the week she left Amadeus, Mastrantonio started on Scarface.

It was learning in public, for high-profile stakes. And, after years of overtures and chorus-lines, movie-making seemed surreal. "It was very, very strange. I'd be doing a scene and I'd just get comfortable. Then I'd turn around and -- the wall had disappeared! I had to learn to rely on just a tiny space right around me. It was a very physical feeling, like falling off a log."

She says it was Al Pacino who helped her cope. The actor, she says, was "unbelievably patient and sweet with me". Often, he had to reach below camera-line, nudging her gently to keep her in frame. "But he gave me so much more than technical help. He's really watching you, really listening. When you're acting with someone like that, it's like having the ultimate partner in a dance."

She draws up her knees and hugs them. "The depth that man brings to his work! There were scenes where he'd start and it would be like 'Jesus! Okay! I'll do it too!' I used to go home at night and just shake. Because I had no idea that's what acting was gonna be."

Nine years and eleven films later, the green actress has become a wised-up screen professional, with a special gift for projecting female intelligence. "On Robin Hood," says Constantine Gregory, "I was absolutely delighted to see Mary Elizabeth. Here, for me, was a class act coming into the show."

Mastrantonio always returns to stage work between her films. She credits it with keeping her "disciplined" and grounded. "I've never found that it jars to go back and forth. In fact, I almost went home this summer to do Stephen Sondheim's Sunday In the Park. But, for personal reasons, I needed to plant myself here. Domesticity has to mean nesting. Otherwise, six months go by, and you don't know where your underwear is!"

Within the next six months, she expects things "to go topsy-turvy again". But Mary Elizabeth Mastrantonio knows herself and what she values. And she's not the sort of actress for whom celebrity alters the rhythm of life. "I don't need much when I work. I don't need friends, I don't need a lover. I don't need a lot of strokes. I just need to know what's going on." She laughs. "I do need, of course, to be told when I'm going wrong. No one's acting can be an exact, 100% science."

But fame has its own imperatives: do more and do it faster. Speak to anyone, talk up your work, concentrate on getting that next job. Mastrantonio says she doubts that celebrity can be a wise career coach. "You're always going to think, 'Should I have done every single thing I was offered?' 'Should I maybe have taken more familiar kinds of roles?' But too much familiarity doesn't help your work as an actress. Neither does too much of your face or too many interviews. In the end, the whole thing will just start to wear on the audience."

As she reaches for her backpack, hair sweeps over her face. Patiently, Mastrantonio shoves it aside for the hundredth time. The shimmering chestnut mass has an obvious mind of its own. But so, in her equitable way, has Ms. Mastrantonio.