PHILADELPHIA -- As Nanny McPhee, the sensible antidote to Mary Poppins, Emma Thompson is snaggletoothed and warty, firmly dispensing gallons of tough love rather than spoonfuls of sugar. As herself, the English actress and author is pearly of tooth and creamy of complexion, mischief beaming from almond-shaped eyes.

"Nanny McPhee Returns," a corking fantasy in which pigs fly and birds do not, hit multiplexes last weekend. Last week Ms. Thompson, screenwriter and star of the sequel to the 2005 charmer, enchanted 100 children at the Free Library of Philadelphia. Stretching and snapping her elastic voice like an instrument, she explained how special-effects wizards made the piglets take wing. Know, however, that the crow perched on Nanny's shoulder is real. She confided that she surreptitiously fed it so it stayed put.

The grown-ups in the audience were more interested in the disciplinary special effects of Nanny M, under whose magical influence quarrelsome brats stop fighting and start sharing. Given the inquisitive children at the library, the adults didn't have a chance to ask their questions.

But prior to her animated session with the youth from Kids for Change, which supports children at risk, and Project H.O.M.E., a shelter program for homeless woman and children, Ms. Thompson, the mother of Gaia, 11, considered how she might disarm a toxic tween. Not that it's necessary in her house. (She is wed to Greg Wise, Willoughby in "Sense and Sensibility," for which she won a screenplay Oscar, mate to her best-actress statuette for "Howards End.")

"Rudeness we don't like," she says, merry eyes mitigating somber precept. "If it happens, stern words will occur," she says. "We don't do punishment. Conflicts can be resolved in a quieter manner." In the unlikely event that Gaia did explode with "Mummy, you are such an 'epithet rhyming with stitch,' " Ms. Thompson would respond, "I suspect you're right. Anything else you want to tell me?"

Still, Ms. Thompson, who is also mother to Tindyebwa Agaba, 23, a Rwandan refugee whom she and Mr. Wise adopted when he was 16, is no stranger to the extremes of filial love and hate. On the corkboard above her writing desk at home in London are two letters from Gaia. "You're the best Mum in the universe," reads one. The other: "Mom, you're the worst mother. You just don't understand."

"They're both right," says Ms. Thompson, amused and happy to be the fulcrum on which Gaia finds her balance. Ms. Thompson's easy child-side manner may be genetic: Her father, actor Eric Thompson, created the beloved BBC children's hour "The Magic Roundabout." In adapting Christianna Brand's "Nurse Matilda" books for Nanny McPhee, Ms. Thompson drew on the Westerns she enjoyed watching with her father. She conceived of Nanny as a Shane of the nursery: a figure who comes to help the household and leaves when her work is done.

At 51, Ms. Thompson is an institution as beloved in America as she is in her native England. Like Meryl Streep, she is universally admired for her spirit and wit. She exemplifies the distinction that George Meredith made when he wrote, "A witty woman is a treasure; a witty beauty is a power." She wields her power wisely. As both an actress and a screenwriter, she creates women and men who are complex, difficult, indelible.

Thompson Inc. is diversified. Admirably so. She can go dramatic ("Howards End," "The Remains of the Day") or comic ("Impromptu," "Much Ado About Nothing"), but her forte is the serious comedy ("Sense and Sensibility"). She appeals equally to adults ("Primary Colors," "Last Chance Harvey") and juveniles ("Harry Potter and the Prisoner of Azkaban," "Nanny McPhee"). Plus, she writes her own scripts.

The graduate of Cambridge University (where she acted with fellow students Stephen Fry and Hugh Laurie, with whom she was romantically involved) began writing for her eponymous 1988 BBC television show "Thompson." But it wasn't until she made "Dead Again," the 1991 film noir with first husband Kenneth Branagh, that producer Lindsay Doran urged her to write a screenplay.

"She helped me, nourished me and mentored me through that process," Ms. Thompson recalls. "I learned about screenwriting at her feet." The result was "Sense and Sensibility," which Ms. Doran, known wryly in Hollywood circles as "the script whisperer," produced and in which Ms. Thompson co-starred. She walked away with a screenwriting Oscar and down the aisle with co-star Mr. Wise.

Since then, Ms. Thompson and Ms. Doran have worked together on "Stranger Than Fiction" and the two "Nanny McPhees" and have become collaboradorers, colleagues who dote on each other.

Scriptwriting is now Ms. Thompson's second career. She is working on a rethink of "My Fair Lady" -- "a kneading of the emotional center," as she puts it, highlighting the relationship between flower-seller Eliza Doolittle and the father who sells her to linguist Henry Higgins.

While "Hollywood has been lovely, welcoming, kind -- and quite generous, considering that I'm a foreigner," there are things that Ms. Thompson doesn't get about the film capital.

"I don't like the body fascism of Hollywood," the expectation "that you have to be this shape or that. Or that you have to be this young. I find it anxiety-producing," she says.

And then, there are things Hollywood doesn't get about her. "They don't get her integrity," says Ms. Doran, who recalls that when "Sense and Sensibility" was about to be released, Ms. Thompson refused permission to publish a "novelization" of her screenplay with her as the author. "Emma had to explain, 'There's a perfectly fine Jane Austen novel, I just nicked it.'"

(Ultimately, Ms. Thompson's "Sense and Sensibility: The Screenplay and Diaries" was published, an invaluable volume with a lovely foreword by Ms. Doran.)

"In Hollywood, when I hear, 'We don't have a class system,' I always laugh," Ms. Thompson says. "There certainly is a star system, something that London wouldn't tolerate."

"In my country, success can be dealt with, but it's something to be vaguely ashamed of. We don't put our artists on pedestals," she says. "And if that happens, the brickbats come flying fast and furious."

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