By SARAH LYALL

Many things about William Shakespeare's life and work are shrouded in mystery, but the mystery of what scholars call his lost years is one of the most intriguing of all. What did Shakespeare do between 1585, when he is known to have left his home in Stratford-on-Avon, and 1592, when he re-emerged in London, described as an "upstart crow" in the account of a rival playwright? And how did he make the enormous artistic leap between his early plays, flawed works like the bloody "Titus Andronicus," and his romantic masterpiece, "Romeo and Juliet," first performed in 1594?

No one knows for sure, and "Shakespeare in Love" provides a fanciful explanation by imagining the young Shakespeare (Joseph Fiennes) as a nearly unknown actor and playwright who finds his poetic voice through a passionate love affair with one Viola De Lesseps, a woman as clever as she is beautiful. That Viola never actually existed, and that she is played by the certifiably American Gwyneth Paltrow (albeit with a plausible English accent), didn't matter to the filmmakers.

The movie, which opened on Friday, is meant to be a high- entertainment mix of fact and fiction, combining a modern sensibility with Shakespearean language and period flourishes, full of the double meanings and sly jokes that characterize the work of Tom Stoppard, its co-writer.

"What's glorious is that so little is known about this period that you're not trapped by any kind of historical circumstance," said John Madden, the director, who is accustomed to reading between the lines of English history, his most recent film having been "Mrs. Brown," which explored the relationship between Queen Victoria and one of her equerries. "At this point, Shakespeare was seen as some sort of hack; his genius was just being recognized," Madden said. "One of the great things is to catch him at this moment, when he wasn't the Bard, when he was just a jobbing actor trying to make his way while suffering from the kind of pressures that we all suffer from. At the same time, he has this genius that he could just turn on like a tap."

What is it like to play a man whose myth now looms so large in our literary consciousness? Fiennes said he soon got over any misgivings. "Shakespeare is often portrayed as a boring, iconic, almost mythical person," said Fiennes, 28. "But I embraced Tom's script because it gives him a warmer, more generous human touch." There was also a darker side to Shakespeare, one that is touched on in the film as the playwright freely lifts plot and character ideas from other people and mines his experiences for his art. "His love affair is real and profound, and he's lost in the chaos," Fiennes said, "but at the same time he's almost a leech, constantly looking for copy and almost making things happen in order to get a story."

The formidable cast also includes Ms. Paltrow's boyfriend, Ben Affleck, as Ned Alleyn, the actor playing Mercutio in the play-within-the-film version of "Romeo and Juliet"; Geoffrey Rush as Philip Henslowe, the theatrical producer; Rupert Everett as the playwright Christopher Marlowe, Shakespeare's archrival, and a roll-call of British character actors: Imelda Staunton as Viola's nurse and confidante; the hunky Colin Firth as Viola's unpleasant fiance, the Earl of Wessex, and Judi Dench as a haughty but feminist Queen Elizabeth I.

The film opens to a theatrical crisis. The wolves are at the door: Henslowe, who runs the Rose Theater, is trying to beat back competition from its rival, the Curtain, but he is heavily in debt and desperately in need of a hit. He hopes that Shakespeare, his current playwright-for-hire, will come through with a play that gives the public what it seems to want: a romance, a shipwreck and some slapstick humor involving an unruly dog. But Shakespeare's play, tentatively titled "Romeo and Ethel, the Pirate's Daughter," is not being written. Bereft of inspiration, Shakespeare is suffering from writer's block, sitting in his garret in London, throwing page after page of his work into the garbage. It is not until he meets Viola and falls instantly, heavily, in love that he finds the muse he has longed for. As the two enter into a steamy, illicit affair -- she is rich and has a fiance; he is poor and has a wife back home -- he finds that words of love and longing and poetry are flowing, as if by magic, from his quill. And the play he ends up writing, "Romeo and Juliet," is different from anything he, or anyone else, has ever produced.

"Shakespeare was doing something radical for his time," said Marc Norman, the Hollywood screenwriter who conceived the plot some years ago (one of his sons gave him the idea), wrote the initial script and is credited as co-writer with Stoppard. "Playwrights writing in that period all adhered to the notion that comedies were comedies and tragedies were tragedies, that a comedy is a story about love that ends in marriage, and that a tragedy ends in death. But in 'Romeo and Juliet,' Shakespeare was starting with a comedy and ending with a tragedy, and nobody had ever done that before."

To Norman, the most interesting question surrounding the play was how Shakespeare was able to leave his more immature work behind and create a piece of such grace and power. "I started to wonder what caused him to do that, what caused him to make that choice," Norman said. "I thought, 'What if he had a love affair with a woman who inspired him in the course of writing the play?"'

After Norman's initial script, in the early 1990s, Tom Stoppard, the witty and erudite playwright who has supplemented intellectually challenging plays like "Arcadia" and "The Invention of Love" with a succession of screenplays ("Empire of the Sun" and "Brazil," among others) was brought in for rewrites (the project then languished in Hollywood limbo for some years, moving from Universal to Miramax). The resulting film is unmistakably Stoppardian, existing on several levels at once, made for a 20th-century audience but full of Elizabethan-era in-jokes and allusions to Shakespeare's and others' works. At times, Stoppard has Shakespeare speaking lines from his own plays as dialogue in his daily life, and at times he interjects very modern witticisms about the plight of writers (Shakespeare is forever being mistaken for the far more famous and successful Marlowe) and the absurd vanity that pervades the theater.

The movie also forms a loose parallel to a Shakespearean play itself, lacing its comic elements with semitragedy, cross- dressing (Viola defies Elizabethan convention and fools even Shakespeare by auditioning for "Romeo and Juliet" dressed as a boy), and even the odd sword fight or two. A number of details are also thrown in for the benefit of those familiar with Elizabethan cultural history. There is, for example, a scene in which Shakespeare practices writing his name and then settles on the now-much-reproduced form of his signature, and there is the emergence of a minor character, a boy named John Webster who plays with rats and enjoys the theater only for its blood and gore. The joke is that he's the same John Webster who will go on to write works like "The Duchess of Malfi," the Jacobean horror play.

But Madden, the director, is at pains to make it clear that knowing about Shakespeare and his time are not necessary to enjoying the film. "The screenplay has been carefully crafted so that everyone can catch up and understand it and follow it," he said. "For anyone who wants to spot the references, they're all there, but that absolutely must not be a requirement for seeing the film. They're a bonus."

One suspects that the need to tone down the erudition to widen the film's appeal might have sat heavily with the clever Stoppard. But he knew what he was getting into when he signed up, he said. "Broadly speaking, in theater the director is there to do his best to serve the writer, and in movies the writer is there to serve the director," he said. "I wear a very different writing hat in movies." He continued: "It's an interesting medium, popular film, because it's not a homogeneous audience. They get quite schizoid, and they don't know whether they're making the film for the critic of The New Yorker or someone who's had a busy day in Newark and wants to relax."

At the same time, Stoppard enjoyed not having to worry about historical accuracy. Among other things, he was free to make up characters that didn't exist, invent scenes that never happened (between Marlowe and Shakespeare, for instance), and play around with the essence of his real-life characters in his own imagination, something he has done in many of his plays (he has also fooled around with Shakespeare before, in "Rosencranz And Guildenstern Are Dead"). "I have a special take on historical accuracy, which is that all supposed historical truths are temporary, meaning they're always there to be modified in the light of subsequent discoveries," Stoppard said. "But this film is entertainment, which doesn't require it to be justified in the light of historical theory."

For people who love the theater, the film also provides ample opportunity to laugh at the vanity and puffery that surround the acting world. "Tom has lived his life in the theater and been around that breed," Madden said of Stoppard. "He has a wonderfully accurate eye and ear for the kind of little fabrications that actors tell each other. It was very nice to have the opportunity to send all of that up from a historical perspective, where you see the beginnings of modern show business developing."

Many of the people involved in the film also found another of its premises -- that a passionate love affair stirs writerly creativity, rather than so exhausting the writer that he spends his days sleeping off his long nights -- very seductive. Stoppard, for one, has always found that to be true. "I think it happily works both ways," he said. "A successful good day's work is a good aphrodisiac, and a successful night's lovemaking is quite an inspiration." "Shakespeare in Love" closes with a neat premise, that Shakespeare continues to draw inspiration from his romance, immortalizing her in "Twelft Night." "That's a very romantic thought," Stoppard said, "that she never grows old and never dies."