Four years ago, director Saul Dibb debuted with the present-day tale of a young, black Londoner emerging from a stint in jail with the dilemma of whether to assimilate into a familiar and destructive gun culture.  “Bullet Boy” was an intriguing topic undertaken, sadly, with a phlegmatic execution.  This year, Dibb returns with “The Duchess,” which, despite stark distinctions in race and class and an 18th century setting, addresses again the vexing theme of the ramifications of a skewed sense of duty.  But unlike its modern counterpart, “The Duchess” is robust, assured and, most usefully, buttressed by a performance from Keira Knightley in the title role which showcases that she is emerging as both a potentially substantive actress and a burgeoning movie star.

Gamine, limpid, and, at times, preternaturally thin on screen, Knightley has the air of an Avedon Harper‘s Bazaar portrait.  She is a mere slip of a girl, when she is wearing a slip. In the six years since her breakout role in “Bend It Like Beckham,“ Knightley has perfected the pretty-in-a-petticoat persona to such a degree that it can appear that she’s hemmed in contractually to a bodice.

So, as the 23-year-old Knightley evolves into adult roles, she could be well served by studying the example of Kate Winslet, whose early career trajectory has an eerie similarity to Knightley’s, but who quickly broke free from the costume-drama constraints.  Knightley appeared in “Pride and Prejudice” at the age of 20; Winslet starred in her own Austen venture, “Sense and Sensibility,” at the same age.  They both were swept up by blockbusters. Winslet, however, was able to follow her titanic success with decisive choices for roles and films outside of the comfort zone of the multiplexes. She seemed determined as an actress to seek out exceedingly intriguing characters in movies which, if not wholly satisfying, felt substantial. With demonstrative roles in the late 90s in “Hideous Kinky” and “Holy Smoke “ and more recently in, for example, “Eternal Sunshine of the Spotless Mind,” and “Little Children,“ Winslet has forged such a powerful resume that she must be considered, along with Cate Blanchett, amongst the elite on any list of the most respected actresses in the world.  Sadly, for Knightley, her massive hit became a franchise and the carnival ride appears to have no end in sight with a fourth version on the horizon, and while she has indicated that she‘s through swanning about, the paychecks on offer could sway even the most resolute thespian.

The role of Georgiana Spencer — a captivating celebrity of  late 18th century Georgian England who became the notorious Duchess of Devonshire, famed for her trendsetting fashion as well as her ardent Whig politics — could serve as a crossroads role for Knightley. It’s a costume drama but with a part presenting her with an opportunity to exude maturity and gravity.  She’s afforded in Spencer a character of a certain determined resistance toward the double standard of the age, where a male gentry’s amorous affairs were tolerated, while a woman’s were verboten.  Knightley emotes, but with a steely restraint, and she inflicts upon the character a genuine emotional tussle.  She’s delicate yet fills her part with flickers of gestures which underscore her torment.  She flashes an assortment of smiles — shy, knowing, beaming — but when she unfurls her Chiclets with conviction she can still deliver a wallop.

All of the recognizable roles of the powdered-wig brigade are in evidence in “The Duchess.”  Ralph Fiennes does yeoman’s work within the rigid boundaries of the familiar role of the cold, disinterested Duke.  He is effectively charmless but it’s a bit deflating that this gormless character doesn’t resort to “In Bruges” pyrotechnics.  Charlotte Rampling delivers an efficient turn as Georgiana’s mother, a doyen of propriety and her place.  It’s almost odd to hear Rampling speaking in English on film.  In her beguiling role as Bess Foster, Georgiana’s best friend, and, later, the object of a very peculiar arrangement, Hayley Atwell unveils a confident portrayal which suggests she will become a favorite of casting directors.  Taking on the “Colin Firth” role of Georgiana‘s dashing paramour — and what other word can one use in a film of this kind for the strapping love interest other than “dashing“ — Dominic Cooper plays Charles Grey with the proper amount of innocent longing and chest-expanding hubris.  The pithy Charles Fox, the prominent Whig politician with whom Georgiana verbally jousts, is performed with suitable panache by Simon McBurney.

Dibb should be commended for a handsome production filmed briskly, and for providing Knightley with the attention and room to luxuriate in her role. He seems like, and it’s not a bad thing, a 1930s director fawning over his leading lady; Knightley repays his adoration by clearly reveling in the role.

But she has reached a turning point, where her talent is evident but doubt remains as to whether she‘ll succumb to typecasting.  She needs an edgy part, an earthy role, and a character like Jane Fonda’s in “Klute” would help prevent the prototype Knightley has had a tendency to resort to from becoming an unimaginative trend. However, if she chooses the safer path, a route where her talent is usurped by box office aspirations or, simply, lackadaisical choices, it may have a domino effect, and one day, not too distantly but regrettably, she could be seen hawking a best-selling, half-hour workout infomercial, “Pilates of the Caribbean.”