She was the Princess Diana of her day. In fact, she was Di’s 18th-century ancestor. Georgiana, née Spencer, the Duchess of Devonshire, was married to a man who did not love her — the Duke of Devonshire, sort of a head honcho of dukes — so she, wicked smart girl that she was, made a life for herself, cavorting with playwrights and politicians, inventing her own fashions, and generally being all-around fabulous. Delicious in a costume-drama-y way that will delight fans of the genre while being as entirely lightweight as the enormous ostrich feathers on Georgiana’s hats, this comedy of propriety gives us a lovely, spunky Keira Knightley (Atonement) as the duchess and Ralph Fiennes (In Bruges) — in a performance that is exquisitely amusing — as the cold-fish duke. I’m sure, however, that we’re meant to take this all rather more seriously than it will let us, what with all the pouty angst about Georgiana’s thwarted love affairs, thwarted friendships, and other assorted thwartednesses. On the other hand, no flick that offers up paparazzi sketch artists can expect to be taken as solemn drama.