From bit player to scriptwriter of choice in the blink of an eye — that is the metamorphosis Julian Fellowes accomplished with Robert Altman’s country-house murder mystery, “Gosford Park.” Now Mr. Fellowes, 57, is back with the book for the
Disney- and Cameron Mackintosh-produced stage musical of “Mary Poppins,” opening Thursday at the New Amsterdam. Like the Altman movie, Mr. Fellowes’s new show, directed by Richard Eyre, reflects an insider’s knowledge of British society, upstairs and down.

It is a domain Mr. Fellowes has explored before, notably in “Snobs,” his satirical roman à clef of the pleasures and perils of social climbing, a best seller in England. “My wife always says, ‘The narrator’s not Julian,'” the writer said recently from his home in London. “But it’s me, though not completely me.”

Parents may sense a similar ambiguity when escorting the children to “Mary Poppins,” which both is and isn’t a replay of Disney’s classic movie of 1964. Yes, Mary Poppins is still the nanny of nannies, starchy yet upbeat, a martinet with a heart, wafted down from the sky beneath her umbrella to sort out the dysfunctional household of George and Winifred Banks and their children Jane and Michael. As the show now spells out, it’s the parents’ lives that are most deeply in need of her services, though the children need help, too. Just wait for the interlude with the replacement nanny, Miss Andrew — the Holy Terror — held up by Mr. Banks as the paragon who made him the man he is. She brings with her a lark in a birdcage, a virtual coat of arms.

No matter how great the score, though, theater professionals always insist, what makes or breaks a musical is the book. Mr. Fellowes demurs.”The musical is the most collaborative art form of all, because it has such a long gestation. Everything’s interrelated. The scene must yield to the song, and the song must yield to the dance. This has to go this way so that can go that way. When it all comes together, it’s a child of many parents.”

But, under pressure, he’ll take credit for this line of the cook’s: “A slave in ancient Rome was on a pleasure cruise compared to my life in this house!”

“That’s me,” Mr. Fellowes said. “And she has another line I love: ‘I’d like to be rich but the good Lord thought otherwise.’ That one was me.” He is less sure, however, of the origin of the tart question Mary Poppins puts to a police officer in search of a missing statue. “‘Have you lost your marbles?’ I can’t remember where that came from. It may not have been me. I think it was Cameron.” He is on firmer ground with the line with which Mary Poppins announces the fall of Miss Andrews: “You are seeing a lark in a cage for the first time and the last.”

That, Mr. Fellowes discloses, is straight from P.L. Travers, author of the original series of “Mary Poppins” books, which Mr. Fellowes loved as a boy and fell in love with anew rereading them in preparation for his work on the musical.

As befalls a professional of his stature, Mr. Fellowes is regularly invited to speak to aspiring writers who may find his history instructive. His screen credits as an actor run to more than 40 feature films and TV shows, peaking with Lord Kilwillie in 16 of the 63 episodes of the BBC’s “Monarch of the Glen.” Steady work, in other words, but no brass ring. For a change of pace, Mr. Fellowes adapted “Little Lord Fauntleroy” and “The Prince and the Pauper” for the BBC. He also wrote a screenplay based on Anthony Trollope’s novel “The Eustace Diamonds” for the actor Bob Balaban, who at the time was interested in producing.

That one did not fly, but he had made an impression, and when the idea of “Gosford Park” was taking shape in the minds of Messrs. Balaban and Altman, Mr. Fellowes was invited first to make up characters and story arcs, then to write a first draft, and so on through completion of the project. “Gosford Park” received Academy Award nominations in seven categories, including Best Picture and Best Director, but its only statuette was for original screenplay. The moral for young writers? “When you write a script, you don’t know what it will do for your career,” Mr. Fellowes said. “If you think of scripts you do on spec as audition pieces, they won’t be failures, even if they never get produced. If they get you something else, that’s success, even if it’s not what you were aiming for.”

Back chez Banks, at no. 17 Cherry Tree Lane, the problems have a remarkably contemporary ring: Mr. Banks can’t show his emotions, and the warm-hearted Mrs. Banks is being stifled by her merely supporting role as wife and mother.

“There’s that ghastly word ‘relevance’ that makes one reach for one’s gun,” Mr. Fellowes said. “But if you can make drama of any period connect with the problems people must wrestle with in the present, the piece will resonate with them.”

To that end, Mr. Fellowes portrays Mary Poppins as the ultimate family therapist, who sheds light in the darkness, facilitates change, and moves on, sailing into the night sky in sensible shoes, feet in first position, umbrella held high, her profile like the prow of a ship, indomitable.

“I would like to think that we have been pretty faithful in transcribing the things she cared about: allowing children to have air to breathe, letting people be themselves, letting them find out who they are,” Mr. Fellowes said. “In one way, it’s the modern sensibility: finding yourself and all that. But in any generation, the successful men and women are the ones who have come to know themselves. That’s what I think ‘Mary Poppins’ is about. I think it’s a moral show. What would be the point without a kind of morality? At the end, the family is cured, so it’s right for Mary Poppins to go off and look for another family in trouble. It sounds sort of mawkish put like that, but I do believe it.”