There's an awful, shaming moment on my set visit to Green Wing, when I realise I've eaten the comedy biscuits. "Oh, my God, we only have one packet," says the unit manager. "And we've only got one banana left for Mac's big scene with Guy. Make sure you take small bites," he tells Julian Rhind-Tutt, who plays Dr "Mac" Macartney, the show's ginger-haired heart-throb.
It comes as a surprise that Green Wing is so low-budget. On screen, it looks extremely flash (Channel 4 has just spent a small fortune on adverts for the new series), but in fact it's shot like a small guerrilla war. "We're not as glossy as people think," admits the series' creator and executive producer Victoria Pile. "The money goes on the large number of people involved."
Today, we're filming in an actual NHS hospital, Northwick Park in north London. The Green Wing crew have rigged up a makeshift set in a corridor next to the Diet and Nutrition department. It's not very glamorous. There is a strong smell of antiseptic and burnt cabbage. Every so often, a member of staff comes out and glares at the actors for making too much noise. "It's a bit like a camping holiday on a budget," says the actor Tamsin Greig cheerfully.'
When Green Wing first screened in 2004, few of us had great expectations. But it turned into the most deliciously surreal show on television. A sitcom/sketch hybrid, it practically invented a new genre with its trip-hop soundtrack, jump-cut editing and absurdist physical-comedy sequences. Best of all, it offered real plots and relationships. Many of us were on the floor over the slow-burn romance between Mac and Dr Caroline Todd (played by Greig).
Green Wing is a "hoscom" that exposes the sheer silliness people use to cope in serious, frontline jobs. The cast spent time shadowing real-life surgeons, and much of that material has ended up in the writing, from consultants answering their mobile phones during operations to doing their ski exercises against the wall.
Of course, in the best workplace comedies, no one actually works. We almost never meet the patients in Green Wing, but here at Northwick Park you glimpse them being wheeled past on trolleys. Pile is desperate to keep them out of shot: "My job is to stop them pointing the camera at a bed or a person in a dressing gown." She says the show could be set anywhere: "Hopefully, it's just a heightened version of the human condition."
Green Wing's team of writers play to the strengths of the actors, from Greig's deadpan delivery to the demented physical comedy of Michelle Gomez as the hospital's staff liaison officer Sue White - "Take this book on dealing with difficult people and fuck off!" Often, whole episodes are lifted from the actors' private lives.
Behind it all is the presiding genius of Pile (best known for creating the award-winning sketch show Smack the Pony) who describes Green Wing as "shallow drama, deep comedy". On set, Pile is fantastically hands-on (just occasionally, you wonder if the director Dominic Brigstocke feels emasculated), but then this is her baby. Before the first series went out, the actors and writers spent three months workshopping the characters - almost unheard of in TV. Filming the new series, they break every fortnight for a week of rehearsals.
When the first series went out, it was very much an ensemble show. This time, everyone wants to interview Greig and Rhind-Tutt. But on set it is Stephen Mangan, as the comic-grotesque Dr Guy Secretan, who emerges as the true star. With film roles in Festival and the forthcoming Confetti, it can't be long before he's up there with Coogan and Gervais.
The rest of the cast are all straight actors, but Mangan is a brilliant improv performer who pushes the script to the very limit with his constant ad-libbing. Guy may think he's a ladies man, but he's a total sexual naïf. We watch them film one toe-curling kitchen scene that culminates in Mangan ad-libbing: "Where is my blancmange in the shape of a ***?" He turns to Pile: "Sorry, was that too much?"
Later she tells me: "Stephen is a magician of words. I'd happily have every bit of invention that he comes up with. He can go one step too far, but I probably encourage that." Mangan, in turn, admits that he often phones Pile from the car after filming - and asks her why she made him do such undignified, childish things. "Because it's funny," Pile replies.
Slapstick is a serious business, of course. Pile compares it to choreographing modern dance. One of her early innovations in Green Wing was to use time-lapse photography to slow or speed up the action - dragging out an awkward moment to a mortifying degree, then rushing us on to the next. Some scenes are so fleeting they feel like hallucinations.
Pile says this isn't just a wacky device. Tired of flat sitcom sets, she wanted to bring the look of film into TV. "In the edit, we played around with starting and stopping time, so you feel like someone walking around in the corridor. We wanted to perpetuate the feeling that you're an observer drifting through time and catching it all. It's mostly shot at eye level: we try to avoid overheads or low angles or traditionally dramatic shots."
Another joy of Green Wing is that, unlike American sitcoms, which are full of ludicrously pretty people, the cast are rather, well, interesting-looking. According to Mangan: "Half the show is about taking the mickey out of people's appearances." There's a running joke in the show that Mangan looks like Donkey from Shrek. Greig says she just acts with her eyebrows, while Rhind-Tutt (who has emerged as the show's totty, slightly to everyone's surprise) is cruelly compared to the Duchess of Cornwall in a future episode - Mangan says he made damn sure that went in.
As you'd expect from the creator of Smack the Pony, Green Wing constantly challenges gender stereotypes. In one scene, Joanna (Pippa Haywood), the fortysomething human-resources manager, flips through an album of Polaroids of herself, chanting: "Younger, older... older, younger, older... siren, hag" (it's the seven ages of women, according to TV).
But the women can be clowns as much as the men - such as the moment in the staff changing-room when we see Gomez and Haywood sizing up each other's breasts with vicious intensity. Greig admits she is faintly terrified of the scenes filmed in Gomez's office. "As an actor, you're in with the dragon and you play by the dragon's rules."
The first series ended with the cast hanging over the cliff in an ambulance, a spoof of The Italian Job. The new one opens with Mac in a coma; when he finally awakes he's completely forgotten his affair with Caroline, re-opening all the romantic possibilities.
Mangan and Rhind-Tutt are still love rivals. "My job is to make him look adorable to women," Mangan groans. "Steve does what he wants, and we do what we can," Rhind-Tutt says dryly. Pile reveals that when they were casting the show, Rhind-Tutt was the only actor able to stand up to Mangan's aggressive, free-form style.
On set, the mock rivalry is played out between takes. "I'd just like to point out that Julian did the whole scene on tiptoe to look taller than me," Mangan sulks. Everyone laughs, but when they play back the scene on the monitor, it's true.
Later, during a kissing scene between Greig and Rhind-Tutt, he hoots: "Look, it's the battle of the noses." Rhind-Tutt wipes his mouth elegantly. "Sorry, that was a bit banana-y," he apologises to Greig. "Concentrate please, everyone," Pile urges. "Remember, we're laughing, crying and getting sexually aroused, all in the same scene. Stephen, I want you to be harder rather than funnier."
Channel 4 is tight-lipped about the plot twists in series two. On set, I spot a blackboard with a list of scenes already in the can: "Dwarf's Wife", "Threesome" and the rather worrying "Coffee Enema". But there's a visible air of tension today, because there's a death on the cards. I can't say who, but it's going to be big.
It's a measure of Green Wing's daring that serious issues are mixed with the slapstick. (Pile's models include Reginald Perrin and Fawlty Towers). It is also curiously innocent. "Caroline is involved with all these men, but you've no idea if she's slept with them. We don't see any sex," Greig says.
Pile should be on top of the world, yet she admits that the critical success of the first series - three Bafta nominations - has been a bit of a millstone. "We weren't set up to write very much more. It's incredibly difficult to expose eight characters in an interesting way each week; plus the actors are enjoying themselves more this time, so there's often a lovely scene on the end or a bit of extra dialogue to incorporate. It can be hard to treat everyone fairly. The second series was quite a nightmare... well, labour of love, I should say."
Rhind-Tutt adds: "This time round, you know the role you're playing, which makes it a bit easier. But at the same time you start to lose your innocence as a performer. I have to watch out because I'm always in the especially cheesy bits." He says that in every scene the camera is trained on all the characters at once to avoid clichéd "reaction" shots. "It ratchets up the comic tension because each scene has to be self-contained. It's a bit like watching 80 tiny pieces of theatre, one after the other."
Pile is nervous about how much is riding on the new series, and thinks the new C4 adverts look a bit smug. Fans may wonder about episode one; it feels like Green Wing: The Greatest Hits, with rather too many dream sequences and cameos with the cast dressed up as sci-fi characters or 1980s rock stars. As for the coma plot-twist - after Life on Mars, it feels a bit dated.
"The problem with the first episode of a new series is that you have to kick off in a way that brings you up to date," Pile says. "We didn't want to spend the whole time recapping. We wanted to get on with the funny stuff and give the characters space to carry on with their emotional journey. We knew we had to start again with the Mac and Caroline story. And the only way we could do that was for him to be unconscious."
Of course, Green Wing is bonkers, but we love the deft characterisation, the sweetness mixed in with the black comedy. Stick with it; the later episodes I saw being filmed looked great. "One of our comedy pursuits is to not look like we're not trying to be funny," Pile says. "We don't do catchphrases; we don't do overt, jokey dialogue. It has to be taken as it was conceived: slightly tongue in cheek. I just hope we're not going to disappoint everyone."
'Green Wing' starts on Channel 4 on 31 March. The DVD of the first series is released on 3 April