Two good reasons for a very hot ticket

Actors don't read reviews, but Keira Knightley must be haunted by the film critic David Thomson's declaration in his New Biographical Dictionary of Film that she acts like a model even when she's not playing one, as she steels herself for only her second stage appearance this week.

And just to incite other odious cinematic comparisons, she's playing the Audrey Hepburn role (check those doe-eyed, crop-haired, gamine similarities?) in Lillian Hellman's The Children's Hour, opposite the American actress Elisabeth Moss, aka Peggy Olson in Mad Men, who has much more stage experience – although she only made her Broadway debut in 2008, in David Mamet's Speed-the-Plow – a ready-made accent, and no nimbus of dissenting critical appreciation.

So, is this the ultimate in celebrity casting, as new Brit film-icon is paired with modishly hot telly-star in controversial lesbian drama?

You might think so, although The Children's Hour, in which there is a whispering campaign that two female teachers are having an affair, is more about the power of slander than sexuality. If it's any good, all cynical reservations will instantly disappear. But if it's a lulu, it will be remembered as the moment Knightley's acting pretensions withered and doubt was cast on the artistic protestations of West End managements.

This, of course, discounts the overriding good sense of casting Knightley in the first place: she's hot, she's young, she has profile. And she has bona fide theatrical genes: her mother, Sharman Macdonald, is a well-regarded Scottish playwright and screenwriter who only set about conceiving her famous daughter, apparently, once she'd achieved her breakthrough hit with her play When I Was a Girl, I Used to Scream and Shout; her dad, Will Knightley, is a splendidly voiced, lugubrious actor who appeared in notable fringe plays in the 1980s.

Los Angeles-born Moss's parents were musicians. And her reputation, unlike Knightley's, which wavers with every movie, is unassailable: first as Zoey Bartlet in The West Wing, and now as Peggy, the watchful, enigmatic copywriter in Mad Men, she is one of the most popular performers on American television; she's less well known in movies, but at the age of 28 (three years older than Knightley), she has plenty of time to make that good.

Knightley also has a new film out – Never Let Me Go, about state-sponsored organ donors – in which she stars alongside her chum Carey Mulligan, who had huge stage success as Nina in Ian Rickson's lauded Royal Court production of Chekhov's The Seagull.

Rickson is Knightley's director of choice on The Children's Hour. Her first stage performance – as the pouting actress Célimène – in Molière's The Misanthrope, was a provisional success in her first move to flaunt stage chops. Her voice was small, her body was thin, but she fitted the role – and the contemporary take on Molière's mildewed media world in Martin Crimp's wittily corrosive translation – to an absolute T.

The Children's Hour is something else. Last seen at the National Theatre in 1994 starring Clare Higgins and Harriet Walter, it is a drama that depends on the protagonists' ability to project complex sexual ambiguities as well as a fierce sense of their own worth, and worthiness, in the face of a slanderous witch-hunt and scare-mongering.

Moss has made pre-opening murmurs about the warmth she's experiencing from preview audiences. And Knightley – who has said that being on stage is like having sex "with an orgasm at the end of it" – is at least to be praised for attempting so difficult and challenging a play.

If we are to have celebrity casting in the West End, in this case it might mean a new, and wider, audience for a play that deserves one, and for actors who have nothing to fear if the public turns out for the oldest, and soundest, reason in the book: they want to see stars they know and like.

'The Children's Hour', Comedy Theatre, London SW1 (0844 871 7622) to 30 April