Behind Downton Abbey, the British series enthralling viewers on both sides of the Atlantic, is the world as imagined by Julian Fellowes: wistful, sharp-eyed outsider and dyed-in-the-wool aristocratic observer. Visiting the show’s creator—as well as its “belowstairs” set and its upstairs, Highclere Castle—David Kamp learns how Fellowes found his calling, why there’s been a Downton Abbey backlash in the U.K., and where the Crawleys and company are (or aren’t) headed in Season Three.