There are many theories to explain the phenomenal popularity of the series-turned-movie Downton Abbey, but Jim Carter, who plays Carson the butler, has one that’s as good as any — Downton Abbey as bubble bath.

“There are so many factors in the show’s appeal, it’s hard to reduce it to any one thing,” said Carter, who blitzed Philadelphia this week with wife and new-to-Downton cast member Imelda Staunton. They were accompanied by Kevin Doyle (a.k.a. Molesley the footman) and director Michael Engler,

They hosted a screening and a Downton-themed cocktail party, greeted fans, and answered questions about the show, in the theater and at the headquarters of Comcast, which owns Focus Features, the movie’s distributor.

“For a certain kind of viewer it offers safety. The audience can settle down and relax their shoulders and know we’re not going to have men bursting through the door with guns [although that actually happens in the movie]. There’s not going to be any violence, foul language, no overt sexual scenes. And here’s something else — everybody in the show is looking for love, so it’s a romantic story. Daisy the maid, Lady Edith is desperate to be loved, Carson finds love — it’s all of these things thrown together in a kind of warm bubble bath of drama.”

Right now the cast and crew are bathing in adulation. Their Tuesday/Wednesday stop in Philadelphia, greeted by enthusiastic fans dressed in costume, is typical of the reception they receive most everywhere.

Staunton, who plays Lady Bagshaw, a new rival for Dowager Countess Grantham (Maggie Smith), has an Oscar nomination for Vera Drake and four Olivier awards for her work on the London stage, so she knows about being the center of attention, but she’s gobsmacked at the reception for husband Jim as he crisscrosses the States.

“It’s like touring with the Beatles,” said Staunton, who has her own theory about why the show connects so profoundly with modern audiences.

She thinks people can watch the movie, or the show, and in that fantasy realm see the kind of leadership — Tory and Labour, if you will — so sorely missing from real life.

“There’s Lord Grantham (Hugh Bonneville), who represents the upstairs, and Carson, who leads the downstairs staff, and you see a picture of two honorable men who endeavor to make the system work. I think people admire characters like that, who respect the people around them, who are compassionate, and who moreover have this profound sense of duty, and you look around you, and you realize these people in the real world are few and far between,” Staunton said.

She was surprised to get an offer to appear in the movie, which came independently of her husband’s involvement.

“I never wanted to be in the series — that was always Jim’s territory completely. He had that [she looks at him a bit enviously], while I was on stage killing myself year after year, every night. But I had visited the set a couple of times, and I’d worked with several cast members before. When I got the offer for the movie, I was shocked, shocked and very pleased.”

She plays lady-in-waiting to Queen Mary, who, along with King George, pays a visit to Downton, throwing the rigidly managed estate into turmoil. Carson is summoned from retirement to mitigate rivalry between the Downton staff and the snooty traveling royal servants, who condescend to the locals.

Romantic subplots abound, Staunton and Smith’s characters are involved in an intrigue regarding inheritance, and there’s even an assassination plot, but most of the movie returns audiences to the durable pleasures of watching heated rivalries play out underneath a hardened veneer of good manners.

“Boundaries can be a good thing,“ Staunton said. “You can operate within those boundaries and it doesn’t make you dull or boring. In fact it can be quite exciting, and I think the show proves that.”

Director Engler put in another way.

“It’s a fantasy, where people are more elegant and well behaved and bound by etiquette. But all of that is tied to a sense of community — it’s not just a set of arbitrary and stiffly administered rules. It’s key to the way people are tied together,” said the director, who helmed several episodes of the sixth and final season, and thought at the time he had presided over the definite conclusion to the series.

“I directed the finale of the series, and I felt like it ended there,” said Engler who, like many in the cast and crew, was a bit leery of a revival.

“You don’t want to be just dragging the thing out of mothballs. And it’s a very tight needle to thread. You know that people are passionate about the drama in a very particular way, and you can’t be ignorant of that, and you have to mindful of that sort of service. But you also have to create a legitimate piece of work that has to exist own. I think we were lucky that Julian [Fellowes, the film’s screenwriter and Downton creator] gave us the template for doing that.”

Doyle, who plays Molesley, said candidly that the actors were rather suspicious that it could be done.

“Even as the series was winding down, there was a strong feeling among the cast that we didn’t want to get into an area of diminishing returns, and we wanted to finish strongly, and Season Six would be a good time to do it,” Doyle said. “Most people would have been happy to end it there, certainly speaking for myself, and we were worried that transferring it to the big screen would be a risk not worth taking. But I think Julian has managed it quite well.”

The movie is pegged to open at a highly respectable $20 million, it cost only $13 million to make, and that profitability means the possibility of sequel is never really off the table.

“I suppose,” Engler said, “if Julian were to come up with something amazing, I could find myself saying, ‘No, I don’t think I’m done with it yet.’”

Doyle is more definitive.

“No,” he said “I think we’re done with it.”