Whatever do they see in each other?

Steve (played by Jack Davenport), one of the two central characters in Coupling, BBC2's clever sitcom about the ever-shifting relationships of six thirtysomethings, is doing what all men do when confronted with the imminent birth of their first child: panicking. He is becoming increasingly hysterical about what to him seem the laughably inadequate forms of pain relief offered to his partner, Susan (Sarah Alexander), while she's in labour. "Is there anything they don't believe?" he snorts with derision. "Breathing! It's right up there with 'I'll phone you next week', 'The internet is a research tool' and 'I'll tap your head a second before'."

Steve, as so often in this series, is exhibiting all the telltale indicators of that classic and incurable millennial archetype: the Rubbish Bloke (RB). Nothing too surprising about that - the RB has become as familiar as England going out of major football tournaments on penalties. But what is remarkable is that the problems encountered by Steve and Susan frequently replicate the problems encountered by Steven Moffat, the show's writer, and his partner, Sue Vertue, the show's producer. Emphasising just how much is drawn from his own life, Moffat has not even bothered to change the characters' names.

Alexander, for one, finds the level of self-revelation in Coupling quite extraordinary. An alluring actress, whose legions of fans pay tribute to her in websites with such titles as "I Genuflect at the Feet of Sarah Alexander" and "The Sarah Alexander Online Shrine", she is also known for series such as Smack the Pony, The Worst Week of My Life and Armstrong and Miller. She plonks herself down on the tatty old sofa in the middle of the church hall in Kensington, west London, that is doubling as a rehearsal room, surrounded by the detritus of the thirtysomething life (mobile phones, glossy magazines and loads of empty beer and wine bottles - all props, I hasten to add). Her bright eyes gleam as she explains how it feels to play out the innermost secrets of a couple who are watching from the other side of the room.

"It's bizarre that Steven and Sue are producing a series in effect about their own relationship," Alexander marvels. "It feels weird portraying scenes that you know must come from their own lives. For instance, when Steve is being so crap about his missus giving birth, I sometimes look at the real Sue and feel very sorry for her, knowing that's exactly what she had to go through.

"Why didn't she just slap him in the face and say: 'Deal with it'? I would have done. I'm not as tolerant as my character."

Davenport - who made his name in This Life and has subsequently starred opposite such actors as Johnny Depp (in Pirates of the Caribbean), Jude Law (The Talented Mr Ripley) and Naomi Watts (The Wyvern Mystery) - is equally flabbergasted by Moffat's candour. "Steven has mined a lot of comedy from his own marriage," he says, almost whistling in admiration for the writer's bravado in disclosing his genetic inability to communicate with the opposite sex. "Quite how Sue feels about him washing their dirty linen in public, I don't know. I'm like a comedic Persil, or the sitcom version of the Shane Ritchie Daz Doorstep Challenge, washing their smalls in public.

"Although we're not doing impersonations of Steven and Sue - the situations are universal anyway - we've stopped asking where real life ends and fiction begins. It's more fun to be surprised. Having said that, when I'm given a Coupling script and it's a combination of sitcom and docu-soap, I can't help thinking, 'Oh my God!'"

Joining us on the sofa to defend himself and eager to avoid a looming deadline on his next script, Moffat is disarmingly cheerful about the fact that every week he is exposing his deepest, darkest thoughts to the entire population of Great Britain. Some of the things he reveals about himself, you wouldn't want to tell your best friend, let alone an audience of several million. It's like the largest public confessional in history. Remember the episode "Unconditional Sex"? The title says it all.

"Jack and Sarah are just a younger and better-looking version of Sue and me," deadpans the writer, a wry, wiry Scot with thick, curly hair, who is going on to pen episodes for the eagerly awaited new series of Dr Who. That's all very well, but why does Moffat draw so copiously from the well of his own life? "I always pour myself into my characters," he says by way of explanation. "Otherwise, I'd have to research and meet other people - and I can't be bothered with all that. But I'm happy to use other people's lives, too."

"That's why I don't say much," Davenport chips in from further along the sofa. "I just sit here quietly with my nose in a magazine."

"Ah," Moffat ripostes, "but it's what sort of magazine that's interesting."

Which brings us neatly to one of the most celebrated episodes of Coupling, when Steve's stash of pornography is uncovered by Susan. Moffat laughs about the incident and describes it as a rite of passage for a couple. "It's an important stage in any relationship when the man's porn is discovered," he says with a knowing smile. But doesn't he mind laying such foibles bare in front of the whole nation? Not in the slightest. "There is no humiliation that I have not now admitted to publicly," he continues, blithely. "It actually gives me a great feeling of liberation. Yes, you found my porn. Yes, I put it on BBC2. Beat that!"

Davenport gives another gloss on what motivates Moffat to offer such a huge number of hostages to fortune in Coupling. "The scripts are like the raging id to the ego of a man who's firmly in his place in the domestic set-up. When you've had a row with your other half, 10 minutes later you always think, 'I wish I'd said that.' There's an element of that in Coupling. Steven just writes down the things he wishes he'd said. It's the longest last word in history, covering 15 hours of television."

So how does Moffat's "other half" feel about seeing her private life up there, paraded on the screen every Monday night for the nation's delectation? Vertue, who is eager to send her husband home to complete his script, comes over to chat with her mother, Beryl Vertue, the executive producer of Coupling and a TV comedy veteran previously responsible for Men Behaving Badly, Steptoe and Son and Up Pompeii.

"At first," Sue recalls, "I did ask him to take out little bits of stuff because they were too personal. I said, 'If you put that in there, the audience are really going to think I did that.' But I've given up now. I've just got used to it. Also, he's so late with his latest script, he can just put in anything if it will help him finish the darn thing."

The producer admits that the scripts have taught her quite a few things about her husband, some of which she may wish she hadn't discovered. "He writes very well about the minutiae of domestic life. He says, for instance, that a man can't go out of a room without a woman saying: 'Where are you going?' Her every utterance is chore-attached. Also, Steven can't understand what cushions are for, and he is unable to comprehend why women need so many pairs of shoes. And I now know never to buy him any small ornaments."

Her mother adds: "When Steven wrote an episode about the need always to have a lock on the bathroom, I immediately called a builder and said: 'Can you put one on before he comes to visit this weekend?'"

The latest series of Coupling, which starts on BBC2 on Monday, further mirrors Moffat and Vertue's life together, as for the first time it introduces babies. "Now there's a little person on the scene, Steven has a whole lot of mewling and puking to write about," Davenport grins.

Moffat, himself the father of young offspring, underlines that he is eager to avoid a rose-tinted view of parenthood. He wants to show the seismic effect on unsuspecting couples of the arrival of sprogs. "Cute is horrible," he says. "There is nothing cute about a baby being born. You're ragged with sleep deprivation, and you're left in charge of a baby when you don't have a clue what you're doing. In most sitcoms, couples have a baby and carry on going out as though nothing has changed. They appear totally unaffected. Why don't they look 20 years older and smell of vomit?

"Also, it's a fact of life that women in labour have the brains of fishes." Turning to his wife, he smiles. "Deny it if you can. You can't let them make decisions about drugs. You might as well ask the foetus. Obviously, whenever I write this sort of stuff, I'm praying other people think the same."

Coupling has struck a chord because viewers connect with the characters. People also tend to have their own favourite out of Steve, Susan, Patrick (Ben Miles), Jane (Gina Bellman) and Sally (Kate Isitt). Richard Coyle's Jeff is replaced in the new series by Oliver (Richard Mylan). Davenport, who is now rushing off to the Isle of Man to star opposite Depp again in The Libertine, laughs at the idea that viewers relate to the characters. "I hope they don't identify with these people - if they do, they should be sectioned. Of the six characters, Steve and Susan are the only ones who are allowed to conduct a relatively normal, adult relationship.

"Everyone has a mate like Patrick, who shags anything with a pulse and gets away with it, and a lot of my friends are like Sally, with an extraordinary capacity for man-hating. And God help anyone if they're like Jane, because she's just bonkers. Having said all that, the characters in Coupling are in a situation that a lot of people will recognise. We're not doing Lord of the Rings here. I'm not wearing pointy ears - although I do have very hairy feet."

With its six central characters who regularly meet in a bar, the show has frequently been compared to Friends. The US NBC network was certainly hoping Coupling would imitate its hugely popular sitcom when it commissioned an American version last year. But the show crashed and burned in the States and was pulled after a month.

Moffat is now able to see the funny side of that escapade. "It enjoyed a triumphant four-week run. Part of the problem was, they gave it a ridiculous promotion. The show had problems, and when it's promoted as the biggest thing ever, there is bound to be a backlash. Before it even comes out, you can't call it: 'The show they're all talking about'. People will just think of it as: 'The show nobody's talking about because nobody has seen it.'"

Sue Vertue says the experience left them "cross, but not bitter". It certainly does not seem to have dented Moffat. He is still able to turn out a smart, funny show that reveals rather too much about his own psyche - but gives the rest of us a rattling good night in. Appealingly, he does not overrate the significance of the show. It's a laugh; nothing more, nothing less.

"Comedy is not a good vehicle for national disasters," he reckons. "I've had a life of affable friends and comfortable chairs. If I'd had a damaged childhood, I could have written drama, but nothing has ever happened to me, dammit. A damaged life is a treasure-box for writers. Unfortunately, there's sod-all in my treasure-box, so I have to write comedy." Moffat foresees a raft of future series about Steve and Susan and their affable friends. "I still have a conveyor belt of non-events to write about."

So what would be a good subtitle for the show, I wonder. Moffat pauses for a moment, before breaking into a slow smile. "Here's What I Failed to do on this Occasion."

The new series of 'Coupling' starts Monday 5 July on BBC2