On a recent afternoon, the skies over Highclere Castle were a brilliant blue more reminiscent of Southern California than the Hampshire countryside, but otherwise things on location at “Downton Abbey” were characteristically English.

That is to say, there were stiff upper lips among cast and crew, despite the fact that the beloved period piece is more than halfway through filming its sixth and final season.

“We’re not in the least bit sentimental yet,” said Hugh Bonneville, who plays the Crawley family’s proud but bungling patriarch, Lord Grantham. The actor was seated beneath a priceless Van Dyck portrait of King Charles I in the dining room at Highclere, a space that, like the offices of Sterling Cooper or the hills of King’s Landing, has become instantly familiar to viewers in this age of celebrated TV drama.

The series, which airs stateside on PBS under the “Masterpiece” banner, has broken out well beyond the niche audience of Anglophiles and costume-drama enthusiasts to become not only the most-watched drama in the network’s history but also one of the most popular shows on American television, full stop. It has racked up 51 prime-time Emmy nominations, and its sprawling cast has twice won the Screen Actors Guild Award for ensemble in a drama series.

The fifth season, which ended with the crowd-pleasing engagement of Carson, Downton’s curmudgeonly butler, to Mrs. Hughes, the estate’s quick-witted housekeeper, averaged nearly 13 million weekly viewers — barely a dip from the previous year. So why end it now?

“Well, you know, life is a process of hellos and goodbyes,” said series writer and creator Julian Fellowes. “We all know that nothing lasts forever, and you want to leave when they’re still sorry you’re going.”

The original plan had been to end the show after Season 5, but executive producer Gareth Neame pushed for one more season in order to tie up the many narrative loose ends. It also feels right to end the series as it pushes into the late 1920s, said executive producer Liz Trubridge. “We’ve hit a time in history when things are changing. We’ve gone a long way from 1912 to 1925; that’s a huge piece of social history that has never been told this way before.”

Sometime in more recent history — circa 2008 — Neame came to Fellowes with the vague idea for a series that would tell the intertwining stories of an aristocratic family and their servants in the waning days of the Edwardian era. Fellowes, who won an Academy Award for his screenplay for “Gosford Park,” was initially reluctant to revisit the upstairs-downstairs milieu but soon sent Neame an outline for what would become “Downton Abbey.”

British network ITV quickly scooped up the project, but an American distributor proved more elusive. Over lunch at Highclere’s tea shop, Neame recalled an unnamed television executive who told him that nobody in the United States would ever be interested in the series. “Downton Abbey” has clearly proved the skeptics wrong.

“The way this story is told, it’s fast-paced. It’s not Charles Dickens or Jane Austen,” said Neame, who argues that the series, despite its exacting period detail and sharp social commentary, is “more soap than costume drama.”

But the show’s sudsiness has been a sticking point for some critics, who have scoffed at plot twists that occasionally strain credulity and a second season that raced through the entirety of World War I in just eight episodes. Many fans were appalled by the sudden death of Matthew, Lady Mary’s husband and heir to the estate, in the third-season finale, but according to Neame it was the only plausible way to write the character out of the series once actor Dan Stevens decided to leave.

Fellowes now considers Stevens’ exit a blessing in disguise.

“We didn’t want Dan to go at all because we all loved him, and if he’d stayed we would probably have created some other kind of issue, you know, they couldn’t have children or whatever it was. But by going, he then gave us back Mary as a character in search of her future, and that becomes more interesting for Michelle [Dockery] to play than just happily ever after.”

“Downton Abbey” has turned its large ensemble cast, including veteran character actors Jim Carter, Phyllis Logan, Penelope Wilton and Lesley Nicol, into identifiable celebrities. “I’ve been acting for about 44 years. So to be in something which is the most successful thing any of us will ever be involved in at this time of my life, it’s odd really,” says Carter, who portrays the punctilious Carson. “Before, it was, ‘You’re that bloke off the telly’ or ‘Haven’t I seen you in something?’ And now I get, ‘Oh, strange to see you sitting down. I thought you’d be serving.'”

For younger cast members, the experience has been equally life-changing — and not just because of the opportunity to act opposite the likes of Maggie Smith. As the spirited Lady Mary, Dockery has blossomed from a promising stage actress into a three-time Emmy nominee.

Though it can be difficult for some actors to move on from popular television roles, Dockery insists she isn’t worried about being pigeonholed as her aristocratic counterpart.

“Producers and directors, they have an imagination. I had a career before ‘Downton’ and I’ve done stuff in between,” said the actress, wearing a fuzzy pink robe over her beaded flapper gown as she sat on a tree stump on Highclere’s vast lawn. “I’m very proud of the show. I don’t think I could ever be annoyed about it.”

Joanne Froggatt, who portrays lady’s maid Anna Bates, is also eager to prove her range. “I feel like I’ve got a lot more to give, and it’ll be nice to be free to do that again, but at the same time, I will cry on the last day.”

Aware that their days together are numbered, the cast mates have begun to organize group outings, like a recent pub quiz in nearby Newbury. It’s hard to get too sad just yet — in part because few outside of Fellowes and his inner circle of producers know just how the story will end.

Laura Carmichael, better known as the perpetually unlucky-in-love Lady Edith, said she’s “dying” for her character to find happiness but has learned the tight-lipped Fellowes “won’t tell you anything.”

There are many other questions to be answered in the season ahead: Will Carson and Mrs. Hughes work together romantically as well as they do professionally? Will Daisy finally decide to leave service and become a farmer or, better yet, a revolutionary? Will the Bateses get a break already?

The sense of anticipation is high all around, said Carter. “We wait for the thud of the script through the letterbox.”