Continued from page 1

And coinciding with his “Downton” duties, he also played the addled Head of Deliverance for the Olympics commission in “Twenty Twelve,” a riotous BBC miniseries that spoofed preparations for the London Olympics.

“There are people who think I’ve been doing nothing for 25 years, and then suddenly I get this role on ‘Downton Abbey,’” Mr. Bonneville says with a laugh. “But I’ve had a really lovely time for 25 years! I’ve played everything from Shakespeare to sitcoms to period dramas to modern serial killers. I consider myself a character actor, and I do love playing different instruments in the orchestra when I get the chance.”

Of course, Bonneville realizes that “Downton” is a good bet for the lead citation in his obituary. He has finally acknowledged it: This show is a cultural phenomenon, not just a fleeting fad. And he has many theories why.

First, the savory writing by series creator Julian Fellowes. Besides, the cast is splendid. The production values are luxurious. And the premise remains rich with possibility.

“This is one of the few settings, alongside a hospital and a police station, where you can legitimately find a real cross section of society under one roof,” Mr. Bonneville says. “But underneath it all, this series is about romance rather than sex, it’s about tension rather than violence, and it’s about family — both the literal family and the staff as family. It explores the minutiae of those social structures, the nuances of the system as to whether someone’s in or out.”

Not that he would want to be part of it. He doesn’t sentimentalize that long-ago era any more than “Downton” does. And yet …

“These days,” Mr. Bonneville says, “we have relationships that are forged, consummated and brought to an end within 24 hours. Back then, the pace of life was slower, and I think we like to breathe out and enjoy that world — albeit for only an hour or so, on a Sunday night.”

Just one more Sunday night, for now.