Since Keira Knightley plays the title role, let’s just get her out of the way. She looks gorgeous, gives a spirited portrayal of Georgiana Spencer (great-great-great-not sure how many-aunt of Princess Diana) and suffers convincingly. She will never win an act-off with Cate Blanchett but does just fine here.
But it’s the other leads in the movie who are most interesting. Ralph Fiennes plays the Duke of Devonshire, who marries Georgiana not for love but because her mother said she could give him a son. Turns out Mama Spencer made promises her daughter couldn’t keep. The duke treats Georgiana with coldness and cruelty and soon takes Lady Elizabeth “Bess” Foster as his mistress, moving her right into their mansion and having them all take meals together, as if he could bring order and civility to their dysfunction.
The interesting thing about Fiennes’s performance is that just when you want to yell, “Bastard!” at the screen, he does something or says a line in such a way that you almost feel sorry for the duke or at least understand why he did what he did. Fiennes also made me go so far as to wonder if the duke was also a victim of the times and the expectations of him. Then I’d go back to hating him. And that’s the brilliance of Fiennes’s work. You can’t peg his character as an outright villain. You might want to because he does some awful things, but Fiennes just won’t let you.
The other character who refuses to be pigeonholed is the mistress Bess, as played by Hayley Atwell. Atwell, whom you might have seen (or not) in the new Brideshead Revisited or Woody Allen’s Cassandra’s Dream, does nuanced, mature work that belies her young age. 
In lesser hands, Bess might have been overshadowed by Georgiana’s glamorous personality, but Atwell gives her a quiet grace and luminosity that threatens to steal the duchess’s spotlight. Just when you want to despise Bess for so blatantly and disloyally stealing Georgiana’s husband, she’d do or say something that makes your heart go out to her instead. It’s quite a feat that Atwell managed to pull off.
Charlotte Rampling also gives a formidable performance as Georgiana’s mother but she doesn’t have to do much to be intimidating. One flash of those steely blue eyes and you’d probably marry a cruel man, too.
Besides the performances, the costumes are also amazing. Every scene is a stunning display of sartorial artistry. Period pieces can be overly long but this one moves at a fast clip and, with so much eye candy, you’ll barely notice the hour and 40 minutes.
Rating: Good
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