How can kissing be this good on the radio?

Passionate kissing is always challenging. It's occasionally plosive and sometimes squelchy; more often, the only sound to be heard is a gasp as one or other party comes up for air. That at least was the conclusion they reached in Studio N41 Bush House, one damp afternoon in June. They were probably right. After all, this was radio where they know about sounds.

Ciaran Hinds as Valmont - tall, roughly-bearded, green of eye and bold of bearing - is embracing Lindsay Duncan as Merteuil - blonde, slender, elegant and experienced. She has the upper hand. She knows the play they are performing better than anyone, save perhaps its writer, Christopher Hampton. He is there too, watching and making suggestions. The great Gordon House is directing: "I think," he says diplomatically to Lindsay, "we could suggest that you might have gone further, but don't want to. Perhaps you could make the word `goodbye' just come out of the kiss. Let's give it a try anyway". They did, and he was right. A little wisp of a sigh escaped Duncan's lips and the scene was complete.

They were busy recording an audio version of Hampton's play Les Liaisons Dangereuses, for broadcast on the World Service and for cassette release in the BBC Radio Collection. "It combines," says House, "wonderful words, enormous passions, characters destroying each other, a very strong story and plenty of opportunities to use silence in a creative way as well - in the emotional frissons which are the reactions to what is being said". House is a master of the use of silence, giving characterisation to what might seem like a simple absence: you can recognise a Gordon House silence anywhere.

Lindsay Duncan is the only member of the cast who was in the original production, playing the wicked Marquise de Merteuil on stage in 1985, and later in the film. Though she thoroughly understands the motivation of the ruthless Marquise, and the desperation born of frustration that drives her to play her cruel games, such knowledge can be a disadvantage. Deprived of the props and gestures of the other media, she finds herself overcompensating, trying to put the whole story into the voice. "And you don't need to do that," she says. "The microphone is so intimate."

Younger, less famous members of the cast find it easier. Emma Fielding, who plays the virtuous Mme de Tourvel, recommends just being truthful, faithful to the words. But then lucky Emma started her career by winning the Carleton Hobbs prize and, with it, a chance to perform in dozens of radio plays: "One day I was in drama school, the next I had an agent, an Equity card and a job! It was a fantastic training, really teaching me how to read scripts and to jump in at the deep end with a play." Her devastating performance as Sarah in The End of the Affair, on R4 earlier this year, was outstanding proof of her mastery of the technique.

Radio plays are turned out very rapidly: this one took four days to record. In such circumstances, the actors really need to trust their director. Gordon House says that establishing that trust is the most important part of his job. House directed Adam Godley, now playing the young Danceny, when Godley was 11. Now, the younger man is warm in the director's praise: "I think he's the best: he's in his element with actors. He doesn't just say louder here, faster there, he explains precisely why. He really gets under the text - and not all of them do that."

House loves this piece. "I think it is one of the strongest plays of the last 20 years - and it's great fun to have somebody like Lindsay in it, who's done it before. I've got a cast assembled here to kill for. You'd never get people as good as this together in the theatre because they'd all be tied up for months with films and things. The disadvantage is that you have to work so fast, making compromises and giving people notes rather than letting them find out for themselves. Some fine actors just can't work that fast; others get there very quickly and then get bored by the fifth take and start to play around."

The major strength of this play - and the reason that it works so well on radio - lies, everyone agrees, in its language. Hampton had three shots at adapting it from the scandalous novel by Laclos, published in Paris to a furious outcry just a couple of years before the French Revolution. "At first I thought I'd write it in 18th-century English," he remembers, "and I'd got quite a way into it when I realised that it wouldn't connect with today's audience. So then I tried making it completely modern, but that wouldn't do either.

"I needed to retain the fact that there is a character who dies of grief - the intensity of that - and the feeling that these people are transgressors, the sense of a society that has reached its apogee and is hurtling towards the edge of the abyss. It's a dangerous novel, because it touches a lot of sore places that will never go away. But what distinguishes it from any other epistolary novel is that it's constructed like a steel trap: it's just the most brilliant piece of architecture. So I worked on finding some equivalent satisfying shape and, in the end, I found this kind of formal modern language, strictly grammatical, and it seemed to work."

The radio version is only slightly different from the stage play. It still retains some challenging stage directions, like "... wiping away a surreptitious tear" or "... a beat, to convey the impression of a chess champion who has just lost his queen". That last would be tough to act in a film close-up; on radio, it must be nearly impossible. Yet it is astonishing how much can be conveyed by sound alone. "It's tricky at first," says Ciaran Hinds, "but after a while you begin to visualise it with a kind of sixth sense, and it just seems to work. I think it's all to do with being truthful". Hinds is mystified as to why he was chosen to play Valmont: "I've so little experience and there are so many fine radio actors about". It's nearly a good question. A better one is: why cast such a handsome hunk when nobody will see him?

Again, House has the answer. This play is concerned with vigorous seduction and savage jealousies. "Ciaran is a terrific actor and Valmont has to have enormous sexual energy. Quite often you cast a radio play and the actors could - and sometimes do - go straight on and do it on stage. There's a physicality about a person which almost is that person and which communicates itself vocally as he speaks".

Or - the thought came to me unbidden - as he kisses.

`Les Liaisons Dangereuses' (World Service) begins on Saturday.