TWO years ago Hugh Bonneville was just another hard-working British actor whose plummy vowels and cherubic cheeks lent themselves to characters good (Bernie, the stockbroker who never gets the girl in “Notting Hill”) and not so good (Henleigh Grandcourt, the aristocrat who sends the girl fleeing in “Daniel Deronda”).

Then he crisped his consonants and stiffened his upper lip to portray Robert Crawley, Earl of Grantham, the lord of the manor in “Downton Abbey,” the “Masterpiece” megahit on PBS. And hearts around the world swooned.

In “Twenty Twelve,” to have its United States premiere on BBC America on Thursday at 9 p.m., Mr. Bonneville trades his Savile Row tailcoat and red spencer jacket for spandex gym shorts and slept-in trousers — all while keeping a straight face — to play Ian Fletcher, the beleaguered head of the Olympic Deliverance Commission. The mockumentary series won the 2011 British Comedy Award for best TV sitcom.

In a recent phone interview from London, Mr. Bonneville spoke with Kathryn Shattuck about the convergence of fiction and reality, and becoming an international sensation at 48. These are excerpts from their conversation:

Q. You’ve been filming “Downton Abbey” and “Twenty Twelve” at the same time. How is it switching gears between Lord Grantham and Ian Fletcher?

A. Same performance, just different outfits. No, actually, they couldn’t be more different. You have the stately galleon that is “Downton” and the sinking ship that is “Twenty Twelve.”

Q. Which character is more like you: lord of the manor or master bureaucrat?

A. Neither. I certainly couldn’t run a big country house, nor could I organize the greatest show on earth.

Q. Are you going to the Olympics this summer?

A. Yes. I went through legitimate means and put down my credit card for a few tickets. So if I get time off from “Downton Abbey,” I’ll be watching the women’s 100-meter finals and the long jump.

Q. Are you athletic?

A. Ha! You asked that in such a sultry voice [lowers his own]: “Are you athletic?” Am I athletic? In my dreams.

Q. Haven’t the Olympics inspired any great fantasy of athletic prowess?

A. I think I held the under-10 discus record at my primary school for about a day. I managed to throw it about two feet, and then a big boy came along the next day and broke it and left me pouting in the concrete circle.

Q. How do you film a series about a real-life event taking place concurrently?

A. John Morton, the writer and director, is a bit of a genius, in my view. He takes real events and nudges them into the absurd. That happened on the night of the first episode, when the Olympic clock stalled on the show and in real life. I was asked to comment in character, so it became a situation where life imitated art, and art imitated life.

Q. You were a well-regarded actor before. Now, with “Downton Abbey,” you have a new level of celebrity. Has life changed?

A. No, I still get pretty much strip-searched going to America. And I still can’t get a seat at McDonald’s.

Q. But they say you’re the new Colin Firth, that you’re a sex symbol.

A. [rather indignantly] I am no more a sex symbol than I am an athlete. That’s hilarious. Of course one is aware that “Downton” has been embraced around the world. But we’re just jobbing actors getting on with it. When “Downton” finishes, I’ll have my thumb out like anyone else.

Q. Maggie Smith plays your mother on “Downton Abbey.” In Season 3 Shirley MacLaine will play your mother-in-law. How’s that?

A. Oh, my goodness. In some of the dining-room scenes I literally had two of the great dames of the world on either side of me. I was in acting nirvana.