The animation techniques used in Winona Ryder's new film, A Scanner Darkly, would seem the perfect medium for an actress who has, apparently, wished she could simply disappear in the four years since she was convicted of theft after a shoplifting spree in Beverly Hills. It is a live-action film, overlaid with an animation process known as interpolated rotoscoping, based on the novel of the same name by the legendary sci-fi writer Philip K Dick. And Ryder is virtually impossible to recognise in it.

Having twice been Oscar-nominated - for Best Actress in 1994's Little Women and for Best Supporting Actress for 1994's The Age of Innocence - her visibility has dropped to a zero. She has not appeared on the big screen since the court case - apart from the poorly received Darwin Awards released earlier this year - nor has she spoken to the media. Until today, that is - and even now, journalists are banned by her publicists from asking her about the case. She has been tempted to speak by sheer enthusiasm for her role in A Scanner Darkly. Co-starring Robert Downey Jr, Keanu Reeves and Woody Harrelson, the film's animation process has the effect of making them all look like graphic-novel characters.

Having learned by personal experience that there's no place to hide when you're a major movie star, it was those very same themes of paranoia, surveillance and fear of being watched that drew her to this unusual movie. "All this stuff is actually happening right now," cautions the wide-eyed actress. "To me it's really eerie how relevant it is, politically and socially. I'm very happy to be part of a movie like that. I think Dick was really on the money when he wrote it, it's amazing what he predicted.

"I think it's a terrifying time in this country and in the world now. But I think my director [Richard Linklater] says something about the more terrifying it gets, the more people find humour and have to find other ways to deal with things. It's not like people just become sheep when things get terrifying. People do rise to the occasion," says the actress, who confesses that she rarely uses the internet for fear of being tracked.

Based on Dick's own experiences, A Scanner Darkly tells the darkly comedic yet tragic tale of drug use in the modern world. The film's offbeat room-mates are all hooked on an addictive drug known as substance D. Ryder plays Reeves's character's girlfriend, who may be hooked on cocaine or may just be faking it in order to get information.

"I feel if we were making this movie forever I don't know if we'd ever fully grasp it," says Ryder. "It's endless. It was scary. Like: 'Who am I?' You don't know who's telling the truth, who's working for whom, who's screwing who over. You never get all the answers. It's like rats in a cage - the government makes people turn against each other by giving them false information, making them confused while getting them hooked on drugs. It's one of the most complex, layered, unusual and challenging pieces of literature that I've ever read - both the book and Richard's adaptation. To me, its ultimately about identity - loss of identity, search for identity - but there are so many different levels," she says.

For Ryder, 34, there is even a more personal connection to Dick, with whom she was familiar through family connections. "I did know some things about him because my godfather, Timothy Leary, was friends with Philip Dick and my father was sort of in that circle as well. I knew that Philip was an incredibly sweet, caring father and that a lot of his paranoia was about the world that he was bringing his daughters into.

"When I was little I met a lot of really interesting, great people. I wish I could remember them all. It was my godfather who coined the phrase, 'question authority', and it's one of my favourites. To question our government is the most important thing people can do right now in the US," she says.

The actress still looks so fragile that you have to stop yourself from saying something sympathetic for fear she may burst into tears. It's clearly awkward for her to voice opinions on some topics since visions of events unrelated to the film enter the room; televised images of a medicated Ryder recorded by CCTV cameras as she picks up items and carries them out of the store.

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There's a shyness and reticence about Ryder that she didn't used to possess. "I have a home here [in Los Angeles], but I'm pretty much in San Francisco now because I have other things in my life. That has always been my home. I'm also involved a lot with my charity work," says the actress, who this morning has come straight from a night shoot, wiping fake blood from her face and head so that now her short dark brown hair is sticking up at weird angles. San Francisco is also where her parents live. She has shunned her old Hollywood pals in favour of rekindling her once close relationship with her family. Her name was once attached to a long list of boyfriends - Johnny Depp, Matt Damon, rockers Beck, Pete Yorn, Soul Asylum's Dave Pirner, Ryan Adams and Dave Grohl - but dating no longer appears to be a priority.

Ryder and her Scanner Darkly co-stars all drastically reduced their fees in order to star in Richard Linklater's low-budget movie, which many critics hail as the first adaptation of Dick's work to honestly reflect the author's intent - his other work having been previously interpreted in the form of glossy blockbusters such as Blade Runner, Total Recall, Minority Report and Paycheck. "I just see A Scanner Darkly as an incredible story," says Ryder. "It's very heartbreaking and emotional, which is why I don't see it as science fiction so much anymore. A bit like in Brave New World how a medicated population is a controlled population.

"For my final in English I compared George Orwell's 1984 with the actual year of 1984, and even back then there was surveillance, taping phones and stuff like that, but now I think it's almost in a way dehumanising - the perspective of watching people like that because they become less human; they become little things walking... from my point of view, its pretty scary."

A career that showed so much early promise, with films like Beetle Juice, Heathers and Edward Scissorhands, has since dissipated to a point where Newsweek commented: "Since 1994, the actress has made just one quality film, Girl, Interrupted." And even then Ryder watched helplessly as her co-star Angelina Jolie took home the Oscar.

If her last starring role was as a shoplifter, then, hopefully, Ryder is today on her way back to stealing scenes the old-fashioned way. She's not the first talented actress to stumble along the way.

' A Scanner Darkly' opens on 18 August