For Emma Thompson, it was a film screening of more than average importance. Around 800 critics were gathered on their deep red seats in a cinema hall in the Venice Lido for the first showing of Imagining Argentina, an adaptation of Lawrence Thornton's novel about the 30,000 people who "disappeared" during the grim rule of the generals in the 1970s and 1980s. This was Thompson's long-awaited, serious labour of conviction, after two years away from film.

Then they started guffawing. To begin with, it was only a few raucous snorts from a selected few. No one on the Lido felt that the scene which inspired the derision was Thompson's fault - she wasn't even in it. But the British press coverage steamrollered into its "the luvvie they love to hate" mode - aided by a photograph, hastily added for later editions, in which she appeared to be on the point of tears. She wasn't.

Another less-mentioned scene got a different response. Thompson, her hair dyed black as an Argentinian journalist, was shackled to a bloodied bench, bent over, gagging for air as she was raped by a soldier from the military junta. The room was stilled. It was raw, almost unbearably painful to watch.

The rape scene was too much and unnecessary, according to Thompson's mother, the actress Phyllida Law. But Thompson insisted it survive the final cut. She felt the kidnap, torture and dumping of the disappeared should be articulated to a generation of cinema audiences who might not know it had happened.

Thompson, arguably Britain's biggest female film star and one of its best screenwriters, has always been political. She was nominated for an Oscar for playing the lawyer Gareth Peirce in In the Name of the Father, the story of Gerry Conlon's wrong conviction for the Guildford bomb and the false imprisonment of IRA bomb suspects. She played a thinly veiled Hillary Clinton figure in Primary Colors. She is active with numerous human rights and aid groups, and has done work for torture victims via the London-based Medical Foundation for 20 years.

But the awkward reception of Imagining Argentina, written and directed by the British playwright, Christopher Hampton, prompted critics to ask whether international stars should dare to make films about the atrocities of a different culture, in a different language to the victims. Hampton responded: "If you deal with an open wound, people are going to react in a very tender fashion."

The question is not whether Thompson's credibility was dented by the film - critics are adamant it was not. "She has always been a very good, intelligent actor. We have seen extraordinary things from her and will see more," said David Stratton, senior critic at Variety. The question, rather, is what direction Thompson will take next. Friends feel she is entering a phase of political maturity which will be expressed through choice of projects, rather than a megaphone.

Slated for not wearing lipstick to a Trafalgar Square protest against the first Gulf war, Thompson was once called the "poor man's Vanessa Redgrave". An executive balks at the comparison now, deciding "she's smarter than Vanessa".

But some wonder whether Thompson, a Labour party member who once came close to posing nude in gold paint on a magazine cover to raise money for the party but thought better of it, will turn the lens on modern Britain.

In November, Thompson returns to romantic British comedy in Richard Curtis's Love Actually, in which she plays the sister of the prime minister, Hugh Grant. Next year, she starts shooting her second screenplay, Nanny Mcphee - her first, Sense and Sensibility, won an Oscar for best adaptation. Nanny Mcphee is an adaption of the "Nurse Matilda" governess stories of the 1960s, now out of print.

She has devoted years to writing a script about the murdered Chilean folk singer, Victor Jara, learning Spanish and touring Chile. But although she is adapting the book by Jara's British wife, the debate over Imagining Argentina could affect the project - another English language take on foreign brutality. Film critics wonder if she will relent and start directing British films; her writer friends say she has always been driven by writing and could move from screenplays to books.

The writer Jeanette Winterson, who cites her as a support to her own work - "someone I can talk to about the big things" - describes Thompson as deeply self-critical. "She has always been active politically. She wants to change the world, make it a better place. I don't see why anybody should be criticised for that. She's not the kind of person who thinks art, politics, life and love should live in separate boxes. She works from her whole senses, that is what's exciting. It is a female thing. Men like to see things in boxes. The slogan, the personal is the political: Emma lives that, not in a sentimental way."

To Winterson, like many writers, Thompson has "a big brain". "Let any of these journalists write a film as good as Sense and Sensibility," said Jim Sheridan, Irish director of In the Name of the Father. "They would get paid a million for their next screenplay if they did, but they couldn't do it."

He recalls Thompson on set as supportive, someone able to withdraw from the film world for long periods and slip back in. "I think we all thought In the Name of the Father would make an impression. She is brave to do this stuff. She is a gentle, strong person, not afraid of jumping into the breach."

She is known for being an actor of extraordinary depth, the kind that can produce real tears when the script demands. Duncan Kenworthy, producer of Four Weddings and a Funeral and Notting Hill, worked with Thompson for the first time on Love Actually, and was startled to watch her turn on tears mid-way through 10 consecutive takes, while another actress was grappling with eye drops.

On the surface, Thompson is on a similar path to her former co-star and British export, Anthony Hopkins. She chooses roles sparingly, occasionally going for big pictures in order to do small ones. There is a sense that her bigger ventures, such as Junior, with Arnold Schwarzenegger, pay for the smaller projects. Still, while she is seen as a national treasure by some critics, producers and directors feel she is an example of Britain's less than healthy treatment of creative women. In Holly wood, she is "adored", as one critic put it, for shunning Hollywood values - namely botox and a teeth job. In the US, critics have rounded on Antonio Banderas for Imagining Argentina and spared Thompson the scalpel the British press willingly wiped off for use.

Perhaps there is an irresistible urge to see the mighty humbled in the British film industry, one executive suggests. This is more acute given the pantomine press treatment of her brief marriage to Kenneth Branagh. Before the marriage, at the height of her theatre and TV work - which included her own series, Thompson, subject of mixed reviews - Thompson was feted as a future star.

Once the ring was on her finger, she was accused of crowbarring herself into his shots. Even now, the relationship turned by the tabloids into "Ken and Em show" is levelled at her work regularly, while Branagh seems to have escaped it.

The picture of Thompson that emerges from the cast and crew of Imagining Argentina is someone who suffers for art and politics. She landed badly while filming an escape from torture camp, ending up on a hospital bed with 10 stitches in her arm and discussing the first world war and the invention of chloroform in Spanish.

Two and half weeks into filming, it was the first time the crew had come together, drinking whisky over her war wounds in a small Pampas hotel. She told the production company: "It meant that I had earned some rights, to have been in pain, to have spilled my blood in Argentina meant that somehow I felt a little less guilty about playing an Argentinian woman. I put my heart, my soul, my blood sweat and tears into this country."

David Aukin, former head of Channel 4 films, met a young Thompson just out of Cambridge and cast her in the 1985 West End musical, Me and My Girl. He sees her as crucial to British film as a midwife to young talent, with many actors producing their best performances with her. "Some actors let the rest flounder while they do their thing. She is a light. She is professional as well as protective."

Lindsay Doran, the Californian producer who commissioned Thompson to write Sense and Sensibility, thinks that she is one of the best screenwriters in English.

"She is driven by a vocation. Writing is hard. Who would do it unless they had to?" Doran said. "She is one of the few writers who has got to see a really good film of their work [in Sense and Sensibility]. It doesn't happen very often."

Life in short

Born April 15 1959, London

Family Married to actor Greg Wise, one daughter Gaia Romilly

Education Camden School for Girls; Newnham College, Cambridge

Career Cambridge Footlights with Hugh Lawrie and Stephen Fry (1982). In West End in Me & My Girl (1985). Moved to TV: Tutti Frutti & Fortunes of War (1986). Film: Howard's End (1992). Wrote screenplay for Sense and Sensibility (1995), currently developing a script, Fast Forward, with Nick Hornby

Awards Oscars and Baftas for Howard's End (1992) and Sense and Sensibility (1996). Rare double Oscar nomination in 1994 for Remains of the Day and In the Name of the Father.

Emma on Emma "It's unfortunate and I really wish I wouldn't have to say this, but I really like human beings who have suffered. They're kinder"

Katie Heslop