Keira Knightley is undeniably beautiful, of that there has been little argument since she first appeared on the public radar with 2002′s Bend It Like Beckham. But since her rocketing rise to global stardom, thanks largely to the Pirates of the Caribbean films, one thing has constantly been in dispute: can Knightley actually act, or is she just another in a long line of good-looking girls who can wear a corset and speak with a crisp middle-class English accent?

Part of the difficulty in assessing Knightley’s acting ability has long been that she has tended to be typecast, taking a series of parts where she has played the slightly posh young woman that she often comes across as in interviews. Where does the actress end and the character begin? After all, even when playing notorious female bounty hunter Domino Harvey in Domino, the film where she’s gone most against type to date, she may have been playing a gun-toting hardcase, but a gun-toting hardcase with the most pristine of English accents from a posh English background. A Jane Austen character with added firepower.

Knightley’s defenders have, to date, mostly pointed to her Oscar-nominated turn in the 2005 version of Pride and Prejudice as proof of her acting mettle – and, more recently, her part in last year’s much-praised Atonement. Yet her die-hard critics have not been convinced, perhaps unable to get past the good looks and accent that have been the hallmark of all her cinematic outings to date.

The Duchess, as the name might imply, is unlikely to silence these critics. Once again, Knightley is playing a posh young woman in a corset – this time the 18th century Georgiana Cavendish, Duchess of Devonshire. And, much like her characters in Domino, Pirates of the Caribbean, Atonement and the more recent The Edge of Love, this is a posh young woman with a bit of an unusual, rebellious streak.

It’s all too easy to write this off as more of the same from Knightley – and, to an extent, it is. Only not only does this particular Duchess of Devonshire provide ample material for a genuinely engaging biopic – combining an unusual involvement in politics with an equally unusual private life revolving around an ongoing ménage à trois with her husband and his mistress – but the character also gives Knightley plenty of scope for some genuine acting. Even though the filmmakers have opted to make this a fairly unsubtle analogy for the Prince Charles / Princess Diana relationship, leaving out many of the subtleties that make Georgiana such a fascinating character, there is still plenty of meat to fill out the standard sumptuous costumes and sets.

In this she is helped no end by one of her most talented co-stars to date, Ralph Fiennes.
Playing the Duke to her Duchess, his subtlety and gravitas is the perfect foil and catalyst, prompting one of her best performances to date. Perhaps it’s that he’s twice her age, perhaps it’s his decades of experience and reputation as one of Britain’s finest living actors, but Fiennes’ understated, naturalistic reservation in his acting seems to have rubbed off nicely on Knightley. It will be interesting to see if she brings any of this to her next role – playing Cordelia in the much-anticipated big screen version of King Lear, opposite an enviable cast of Anthony Hopkins, Gwyneth Paltrow and Naomi Watts. If she does, even though it will be yet another posh period piece, her critics may just have to bite their tongues for once.