The man who kisses Keira, nightly

In a West End attracting ever more movie stars, and ever more financially reliant upon their fame, it is a pertinent question. A sizeable proportion of the audience at the Comedy will have bought tickets solely in order to see Knightley, after all - and we're suckers for every small detail.

You'll probably recognise Menzies from Spooks, or Casino Royale (in which he played M's aide Villiers) or as Brutus in the HBO epic Rome. You may even remember him from the National Theatre's production of The History Boys. As an actor used to appearing alongside big-name stars, he takes Knightley's fame in his stride.

"I'm largely interested in people who are just great actors, and they're not necessarily hugely famous," he says, citing Mark Rylance as an example. "Do I think the West End relies too heavily on star names? Yes, I do, and it can result in miscasting and sub-standard stuff. Not always, but occasionally." For the record, he describes Knightley as "a very down-to-earth, hard-working actress, to her credit", and reckons, obviously, that The Children's Hour proves how "successfully" a straight play can be sweetened by star names.

Art house-savvy Londoners, meanwhile, will see a lot more of Menzies next month in the British indie romance Forget Me Not - a movie set entirely over the course of one night in an enchanting and seedy and beautifully-shot London cityscape. He co-stars with Genevieve O'Reilly but it's a film in which the town itself takes centre stage. "London is like the third character," he enthuses. "It's a charming, bitter-sweet sort of love story. Those were very happy days, making it."

Less happy was his brush with tabloid fame five years ago, when it emerged that the actress Kristen Scott Thomas, more than 10 years his senior, had left her husband for him. The phrase "toyboy lover" was used more than once, but the affair didn't last.

Now 36, Menzies is serious and charismatic and surely destined for bigger success still. Perhaps because he "stumbled" into the theatre, almost on a whim, he does not seem at all luvvie-ish. "I'm quite realistic about this job," he says. "I've been really lucky to have had a variety of roles, and I don't think I'm in danger of being typecast as the romantic lead. I think there's honour in working as constantly as you can. That isn't easy. And I'm no matinee idol. I live or die by how well I act."

Born in north London, he went to the arty, liberal Frensham Heights school in Surrey when his parents, a teacher and a BBC producer, separated - but acting was hardly on his radar back then. All he knew was that he didn't want a conventional career in an office. "I did stumble on a natural vocation. My mum took me to the theatre quite a lot as kid but it wasn't a particular passion. I suppose I followed my nose in quite a chaotic way, and was lucky enough to find something I was good at." With a starring role in the West End, plus the lead in a new British film, does it feel like his moment? "Here's hoping," he grins.

The Children's Hour is booking until May 7.