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• Cultural, medical and other period tidbits, which are fascinating and a reminder that wise historians never would choose to live in a time before their own. In one instance, a character who may have cancer is told that test results will take up to two nerve-shattering months.

• A stately house, but fast-paced action. Mr. Fellowes said he took a cue from the American mash-up approach to storytelling perfected in shows like “ER” and “The West Wing,” with stories big and small, sad and funny and “all sort of plotted up together.” The look is period but the energy is “much more modern,” as Mr. Fellowes put it.

But modernity can be troublesome, proof being the Internet imperiling the drama’s surprises for U.S. viewers. Whatever the outcome, Ms. Eaton said “Masterpiece” will tread carefully in making changes.

ITV is the primary funder of “Downton Abbey” and has international premiere rights. While a September debut fits the U.K. TV marketplace, it would mean stiffer competition for “Downton” as U.S. networks launch their fall slates, Ms. Eaton said.

“We want to make sure we don’t do something with ‘Downton’ that will hurt it in the long run,” she said — which, for now, extends to the drama’s fourth season set to air on “Masterpiece,” its co-producer with Carnival Films.

As for the current run, Ms. Eaton, who’s no spoilsport, had only this to say: “I think it’s the best season yet.”