In a year when the big promotional push was for “Upstairs Downstairs,” it was the upstart British series “Downton Abbey” that all but took its thunder, becoming the biggest hit in years on “Masterpiece Theatre.”

With a second series set to start Jan. 8 on PBS, its stars and producers visited the TV Critics Association summer press tour over the weekend, still trying to explain why it got so popular.

Producer Gareth Neame figures it’s the mix of a traiditional “quintessentially British genre… this sort of country house, historical drama” mixed with modern writing of “Julian Fellowes’ outstanding writing, written in a very contemporary narrative, with original stories, with this extraordinary ability for him to spin like 20 spinning plates all at the same time.

“It’s very modern, contemporary storytelling, but in a period setting,” Neame says. “And I think audiences love this sort of social interaction, the way that these people lived under a different code from the way that we live now, the manners and the rules that they all you know, they’re somewhat absurd to all of us, but yet we’re quite intrigued by them.”

In all of that, it’s more like “Mad Men,” where “you have a period setting, but modern writing, than it is to a lot of the former ‘Masterpiece’ productions like ‘Pride & Prejudice’ where you have a Victorian novel that is adapted and the screen writer has to be, to a greater or lesser degree, has to be quite faithful to the novel that’s been laid down. We have all the freedoms that any modern narrative would have.”

A standout in the first series was Michelle Dockery’s Lady Mary, who scorned the love of one man and made the mistake of making love to a Turkish prince who died in her bed.

She was an enigma, Dockery says. “On first reading Mary in the first episode, she came across as a very cold person and very much a snob.”

But after the Turkish tragedy, she adds, “I think Mary became a lot more vulnerable from then on. And I guess I felt like she softened a little more, because something had actually happened to her. And I mean, she’s an incredibly complex character.”

When the second season comes, she will still be pining for Matthew, whom she let get away and “she finds out very quickly that he’s moved on to someone else. But that affection for him is still very much there.”

Neame said in crafting the first season finale that flummoxed some, the idea was to have “Enough things to settle so you felt there was some resolution. Enough things were kept open.”

Ending with the outbreak of war, he said, “seemed like a very natural place to end things,” says Dan Stevens, who played Matthew. “It was literally the last moment where things could never quite be the same again and it was I quite like the fact that there’s not a great deal of resolution with a lot of those storylines.”

“And you see this beautiful world all poised on the precipice of cataclysmic change,” says Elizabeth McGovern, who has a great comeback in her role as lady of the house, Cora Crawley, the Countess of Grantham. The effect of the end, she said, was: “Pow, it’s over. Tune in next time.”

In addition to a new love interest for Matthew, there’s one for Mary as well, Naeme says. “There’s a new housemaid. There’s a new male servant.”

“There’s a different dog,” says “Masterpiece” executive producer Rebecca Eaton.

One character not returning is one of the housemaids, Gwen. “She gets a job as a secretary, if you remember,” Naeme says, though he adds, “I think we I think we will bring her back at some point in a secretarial role as a professional woman.

“I hope we’ve got the balance right,” he says of the cast changes. “You will have a bit of turnover, new life coming in, but without the whole thing feeling radically different. I think we’ve got the balance about right.”