Apart, perhaps, from Goldie Hawn, it is difficult to conceive of an unlikelier actress to play the tortured Plath, who committed suicide in 1963 while married to Ted Hughes, now poet laureate.
But according to industry gossip, the star of When Harry Met Sally and Sleepless in Seattle is fixated by Plath and has ordered numerous copies of Birthday Letters, the recently-published volume of poems in which Hughes finally gives his perspective on their turbulent marriage.
Ryan's commitment to the venture represents not only a professional volte- face. It also confirms suspicions that her carefully cultivated off-screen persona - cutesy, vulnerable, unsophisticated - is just a facade and that she is, in reality, a tough businesswoman. For one thing, she is aware that Hughes jealously defends Plath's memory and that, as executor of her literary estate, he has taken robust action against such works as unauthorised biographies. She has, reportedly, assembled legions of lawyers to fend off any attempts to block the film.
Ryan, 36, is now far more than just a bankable actress. Through her own production company, Prufrock Pictures, which she set up in 1992, she has become a Hollywood power-broker. She is writing the script for the Plath movie herself. "What Ryan wants, she gets," said one industry insider.
A friend of Hughes said yesterday: "I can't imagine that he will be overjoyed by the prospect of this latest film."
For all that she gushes in interviews - about her wonderful husband, film star Dennis Quaid, her adorable son, Jack Henry, their 200-acre ranch in Montana - there are subjects that she refuses to address.
Her mother, Susan Jordan, for instance. Ryan has not spoken to her for nearly 10 years, not even when she underwent surgery for breast cancer. Jordan, who has never met her grandson, said in a bitter newspaper article a few years ago: "The image she (Ryan) has of the innocent, dizzy girl- next-door could not be further from the truth. In real life, she's a cold- hearted, cruel manipulator." The gossipmongers say she never forgave her mother for leaving the family home in Fairfield, Connecticut, to pursue an acting career, leaving her husband, Harry Hyra, to bring up Ryan - then known as Peggy - and her three siblings.
Last year, she told an interviewer: "I admire Sylvia Plath because she was fighting the good fight in her head, trying to survive day by day." Ryan will not give up her project without a fight.
MOST MEMORABLE SCENE
In When Harry Met Sally, Ryan electrified a crowded New York deli by faking an orgasm. However, she has never taken her clothes off professionally. "I've got an old-fashioned view of sex on the screen," she once said. "I don't think you need to see naked bodies writhing around."
MOST UNLIKELY ROLES
In The Doors, Ryan played Jim Morrison's druggie girlfriend. She was equally unconvincing as a Gulf War officer in Courage Under Fire, as an alcoholic in When A Man Loves A Woman, and as a deranged biker in Addicted To Love.
LADLE ON THE SYRUP
On motherhood: "I spend so much time changing nappies and I love it.
On Dennis Quaid: "It's easy to be in a marriage with someone who does what you do if you respect him, and Dennis is an amazing actor."
Quaid on Ryan: "She's vibrant and positive."