ONDON -- WIT," the Pulitzer Prize- winning play about a professor of 17th-century metaphysical poetry who is dying of ovarian cancer, is by no stretch a comedy. But it was the humor in Margaret Edson's writing — subtle, dry, turning on a word or a phrase — that struck everyone involved in adapting "Wit" for television.

"Maggie Edson's play is wonderfully written and so smart and layered," said the producer, Cary Brokaw. "In the adaptation, we were in full agreement that we wanted to mine every bit of humor we possibly could. While the subject is inevitably that of a woman dying of cancer, this is a woman who appreciates humor and wit to her last breath. It's at once a story of a woman dying, but also coming to terms with what it means to live."

In casting the lead, Colin Callender, the president of films at HBO (where "Wit" will have its premiere on Saturday night), turned to Emma Thompson, who has that seemingly indigenous British ability to register irony through the slightest sideways glance, the subtlest twitch of her lips, the merest inflection. That, at 41, she is younger than Dr. Vivian Bearing, the English professor who is the protagonist of "Wit" and who is meant to be 50, was not an impediment.

"I thought it was important that the actress was old enough to feel that she'd experienced life and that there were opportunities she might have missed, things she could have done differently, but whether she was in her 40's or 50's or 60's didn't matter," Mr. Callender said.

Why didn't he cast Kathleen Chalfant, who played Vivian when the play opened in New York in 1998, or Judith Light, her successor in the role?

Partly, it was a nondecision: in order to go ahead with the project, it was necessary to secure a star of Ms. Thompson's stature. Beyond that, the filmmakers said, they were eager to make a film that would stand on its own.

"If we had cast Kathleen Chalfant or Judith Light, it would have felt like filming the play," Mr. Brokaw said. "But we made a conscious decision to not do that, but to give it new life as a film while capturing all the qualities of the play we so admired and responded to."

In any case, Ms. Thompson immediately agreed, and almost as quickly enlisted the director Mike Nichols, who had directed her in the film "Primary Colors." The two then turned to a common friend, the playwright Harold Pinter, to play the small but crucial role of Vivian's father, who appears in a poignant flashback in the middle of the film, introducing a 5-year-old Vivian to a lifetime of sometimes overly pedantic scholarship by telling her what "soporific" means.

The film was Ms. Thompson's first project since she gave birth to her first child, a daughter, Gaia, with her companion, the actor Greg Wise. She and Mr. Nichols began rehearsals some months before filming began, debating endlessly how to portray the highly intelligent, sometimes infuriating, but ultimately deeply human Vivian.

It was the challenge of interpreting such a character that drew Ms. Thompson to the part, she said in an interview in her office, across the street from her handsome townhouse in south Hampstead.

She found the material highly appealing. Thanks to her classical British education and Cambridge degree, she was already familiar with the John Donne poems at the heart of the work, particularly Holy Sonnet Six — "Death be not proud" — which takes on a deeper meaning for Vivian as her own death approaches. In her final moments, she is visited in the hospital by her old mentor, a formidable Donne scholar played by the British actress Eileen Atkins, giving rise to a scene in which the two turn Donne aside in favor of something much simpler.

"One of the joys is to watch how Emma and Eileen Atkins bring John Donne to life so it adds a whole new emotional layer to the movie," Mr. Callender said. "In their confident hands, it allows you to understand how the poetry relates to her life, even if you are not familiar with it."

The film begins with Ms. Thompson lying in bed, bald and vulnerable in the thick of her cancer treatment, explaining wryly to the camera how she got to this point. Because the film is set entirely in the hospital, except for a few important flashbacks that help fill in Vivian's past and explain her present, Mr. Nichols and Ms. Thompson agreed that it was crucial to the film's success to get the pitch of this first scene just right.

"We took a long time to try and find the tone of that first scene to camera, to try to get where we wanted," she said. "We talked and talked and talked, and took our time, and we shot it twice — at the beginning and at the end. And it changed. The point about this film is that it's a question of very subtle brush strokes."

Adapting the play enabled Mr. Nichols to give the material an intimacy — the viewer can easily imagine herself sitting beside Ms. Thompson's hospital bed — that wasn't possible on stage. "The play was brilliantly done and technically very difficult, and the material lent itself particularly well to being a film," Mr. Nichols said by telephone from New York. "The script is very interior. There are lots of places to go, both in the head and in the past and in the hospital."

Ms. Thompson agreed. "The script is interpretable in lots of different ways, and on stage I'd have had to play it completely differently," she said. "On stage, you don't have the advantages of film: you don't have close-ups; you can't whisper and go very quiet."

LIKE the actresses who performed the part on stage, she decided to shave her head rather than wear a bald wig. "I have no attachment to my hair whatsoever," she said. "I've always had to have so much done to it — it's been dyed and crimped and permed and cut off and things have been added to it, and it's just part of the job." She is now wearing her hair in a one-inch buzz-cut. "I like having no hair," she said.

She also put self-consciousness to one side for the scene in which several medical students prod her stomach as part of a lesson about the side effects of cancer. ("Vanity?" she said. "Well, you are being paid.")

Ms. Thompson has known many cancer victims, including, in a cruel twist, a friend who died of ovarian cancer as "Wit" was being filmed. And she said she can understand Vivian's strong inclination to die alone, without help — even if much of her intellectually rigorous solitude seems to fall away by the end of the film.

"She's learned a lot about compassion, and a lot about herself, and a lot about her ability to deal with people who aren't as clever as she is," Ms. Thompson said. "She's learned, as she says, that just being clever isn't enough. That remorse is clearly felt, but she can't find a word for it. It is regret."  

Sarah Lyall is a reporter in the London bureau of The New York Times.