WHEN WINONA RYDER FINALLY MET Susanna Kaysen, the writer she would be playing in Girl, Interrupted, the movie star was very nearly speechless. "She said. 'lt's you ... I'm you ... you're me... I'm me!'" recalls Kaysen, who wrote the best-selling 1993 memoir on which the film is based. Like the movie, the book begins in 1967 when Kaysen, then 18, was diagnosed as having borderline personality disorder after she swallowed 50 aspirin. In the wake of this suicide attempt, Kaysen was whisked off to McLean Hospital, near Cambridge, Mass., where she spent two years in a ward for teenage girls. "Of course, I was sad and puzzled," Kaysen writes. "I was 18, it was spring, and I was behind bars."

But Kaysen wasn't sure if she was insane or simply suffering from a bad case of late-teen angst, symptoms of which include a fragile self image, moodiness and uncertainty about the future. All in all, it's a situation to which Ryder could relate. Six years ago, when the actress first read Kaysen's book, she was reminded of a similar breakdown in her own late teens: After having chronic anxiety attacks, depression and insomnia, she checked herself into a psychiatric hospital for a week and began working with a therapist. The experience, though troubling, eventually helped Ryder bring depth and resonance to the character of Susanna.

"Winona's own memory of what it's like to feel alienation added gravity and a sense of purpose to her work in the film," says Girl's screenwriter-director James Mangold, 36. "Significant aspects of her personality–her sensitivity, the way she is so affected by things–fused well with aspects of Susanna's. The movie is about who Winona is as much as it's about who Susanna is."

Still, Kaysen, now 51, was initially skeptical about the movie. When producer Douglas Wick approached the writer about buying the film rights to her memoir, "I thought he was crazy," says Kaysen. "It's not a narrative. It's rather static and intellectual, and it has no obvious story line. How could they make a movie from this? I figured they would have to do radical things to it; otherwise, it's just a bunch of girls sitting on the floor smoking cigarettes."

Indeed, in adapting the book for the screen, Mangold decided to craft a cohesive, chronological narrative and to intensify the relationships between the characters. Most crucially, he fleshed out Susanna's friendship with Lisa, a charismatic sociopath (played by Angelina Jolie) who serves as a symbol of rebellion and escape from the cares of the adult world. "In facing adulthood, Susanna can go two ways," Mangold explains. "One is the way of the pod people in her parents world, marching off to war or college or family life; the other is Lisa's way of seductive freedom." Kaysen didn't mind the changes. "If someone had told me what to do in my book, I would've killed thern," she says. "But the movie is another endeavor, a variation on a theme. They can't change what I wrote or my experiences in my life."

Those experiences have been, by and large, internal. Kaysen has lived a fairly sedentary existence, as perhaps befits a writer. She has lived in Cambridge most of her life, including a brief stay on a commune. Although her father, Carl Kaysen, was an Ivy League economics professor, Kaysen resolutely avoided college, drifting instead from job to job, making a living as a copy editor and writing all the time. In 1987, she published her first novel, Asa, As I Knew Him, about a woman imagining her old flame's youth, and followed it up with 1990's Far AfieId, about an anthropologist doing research in the North Atlantic's Faroe Islands. According to Kaysen the genesis of her memoir actually stemmed from her work on the latter novel: "There were paralels between being dropped into a foreign country and being dropped into the loony bin," she says. These days, Kaysen still lives in Cambridge and suffers from depression, but says she has made peace with her demons, periodically seeing a psychologist, whom she calls her "tuneup woman." She has also made peace with being alone. Although she was briefly married in her 20s, she has been single for most of her life. "I've been looking for a date for 10 years," she admits, a fact that success hasn't changed. "Fame, she says, "doesn't bring a woman in America a date."

SARAH SAFFIAN