A round-headed, round-cheeked man with a sparkle in his eye and a bouncy British accent, Fellowes laughed lightly as he told the story. “And I said, ‘You really don’t need to pray for them, because these actors have never had it so good!’ ” The woman failed to see the humor, telling Fellowes she was particularly worried about Cora, the American-born Countess of Grantham. “So I said, ‘Well, pray for her, it won’t do any harm.’ ”

Watching the eloquent Fellowes talk with open enthusiasm about being at the epicenter of an international TV sensation is like watching an excitable grad student in literature debate his favorite books. “Downton Abbey” has broken records for ITV in the United Kingdom and for PBS’s “Masterpiece” in America; it has captured the support of Emmy and Golden Globe voters; superfan Michelle Obama ordered advance copies of season 3, which begins in the States on Sunday night at 9; and, in what may be the best gauge of success, it has spawned countless online spoofs, including Fellowes’s favorite, “Downton Arby’s.” But still the 63-year-old man, sitting in an empty bar on Commonwealth Avenue on a recent afternoon, displays none of the self-importance that plagues so many Hollywood writer-producers.

“Downton” addicts are praying for his ensemble, a castle full of emotionally stunted aristocrats and servants facing the end of a class system together, and that makes Fellowes giddy.

The oddest moment of “Downton” fever, he says, may have been the night he told a British Film Institute audience that no, he wouldn’t write a novelization of the series, that the only novel you could write without distorting the story would be a prequel. “By the time I got home,” he says, “there was a message from my agent in California saying, ‘I didn’t know you’d written a prequel.’ By the following morning, the blogosphere was convinced that the book was already written, they were setting up the miniseries, and it was cast.

“Those moments, you get such a flash of the way ‘Downton’ is in people’s conversation — in the national conversation, that phrase that I rather like. I do think [a prequel] is quite a good idea, though. And I might do it. But right now I’m spoken for. My dance card is full.”

That’s an understatement. Fellowes, who joined the House of Lords in 2011, is currently in the middle of many projects — among them season 4 of “Downton,” a musical theater adaptation of “The Wind in the Willows” in London, a big-screen redoing of “Gypsy,” and an adaptation of “Romeo and Juliet” with Hailee Steinfeld.

He’s also developing a series for NBC set in 1880s New York called “The Gilded Age.” Not surprisingly, network TV wants to borrow some “Downton” mojo. Imitation, as Fred Allen put it, is the sincerest form of television.

By way of explaining the “Downton Abbey” phenomenon, “Masterpiece” executive producer Rebecca Eaton tells a story about Fellowes. When he was a boy, the son of a diplomat, his mother would let the kids take over the kitchen every now and then. “One year, little Julian decided to make éclairs,” she says. “And he made them and they were fantastic and everyone gobbled them up. And his mother said, ‘Julian, they are so good, how did you make them?’ And he said, ‘I DON’T KNOW!’

“I think [the success of ‘Downton’] was as much a surprise to him as to the rest of us.”

That hasn’t stopped every culture vulture from West Hollywood to the West End of London from trying to dissect and explain the triumph of the series. Why has this particular production proven that the costume drama is still viable as mass entertainment, and not the many fine “Masterpiece” miniseries of recent years? “Downton” has no brand-name advantage — no Charles Dickens or Jane Austen byline and no major-star appeal. When Fellowes was in pre-production with “Downton,” the BBC announced it would be reviving the similarly themed 1970s classic “Upstairs Downstairs,” and Fellowes and the other “Downton” producers had a “crisis meeting,” he says, afraid they’d already been sunk by a more familiar product.