Stephen Mangan is on his fourth glass of champagne and it is barely lunchtime. The tousle-haired Green Wing star does not have a drink problem, of course. The fizz is a non-alcoholic prop. He has been filming a restaurant scene for Bitter and Twisted, a one-off sitcom to be aired this autumn. At last there is a cry of “Cut!” and the busy actor can relax. “Humour is at the centre of our culture,” Mangan suggests when asked why he enjoys making comedies so much.

Bitter and Twisted is part of Comedy Showcase, a refreshingly forward-looking celebration of 25 years of Channel 4 comedy. Over six Fridays there will be a sketch-show pilot for the Star Stories frontman Kevin Bishop and five sitcoms: Other People, starring Martin Freeman as a disgruntled magician; a Victorian house-share spoof, Ladies & Gentlemen, with Reece Sheersmith; an Irish comedy, The Eejits; a romance, Plus One, featuring the former EastEnder Nigel Harman; and Bitter and Twisted, set in the world of showbiz agents.

It is an ambitious, admirable project, as Andrew Newman, Channel 4’s head of comedy and entertainment — and closet Brittas Empire fan — understandably agrees. “If you get a sitcom right, it’s a fabulous thing to behold.” Bitter and Twisted certainly seems promising. Mangan and Sharon Horgan, his co-star, look good as the hesitant lovers with dark pasts — she is mourning her dead fiancé, he is recently divorced. The milieu is one that the writer, Chris Niel, certainly knows about — he was an agent for Matt Lucas before changing careers.

No one sets out to make a bad sitcom any more than they would set out to open a bad restaurant. But like dodgy diners, there are lots of forgotten flops. Who remembers that series set in a workplace staffed by losers, misfits and a podgy middle-aged boss? No, not the one written by Ricky Gervais, but Nice Day at the Office in 1994, starring Timothy Spall. For every Fawlty Towers there is a Sam’s Game (starring Davina McCall), for every Blackadder a Heil Honey, I’m Home, following the crazy antics of Adolf Hitler and Eva Braun. Well, maybe it looked good on paper.

So is there a science to sitcoms or is it the same as the movie business, where, as the screenwriter William Goldman once revealed, “nobody knows anything”? Henry Normal, Steve Coogan’s creative partner, recently said that writing comedy is somewhere between mathematics and music. Newman agrees — up to a point. “You couldn’t get a computer to write a sitcom, but there is a structure to them. Yet the great sitcoms come out of a hard-to-put-your-finger-on mix of writing, production, direction and stars.”

There are clearly plenty of ways to mess up a sitcom and far fewer ways to make a classic. Horgan knows about the subject too, having created the cult BBC Three hit Pulling and Five’s forthcoming Angelo’s. “The secret is when something is heavily auteured, when it is written and directed by people who care about it and share a vision and don't let anybody else s*** on it. And when the channel trusts you and lets you get on with it, like with The Office. But then I could be wrong.”

Sitcoms are the biggest of telly gambles. Concoct a winner and you have beaten the bank. “They are a really high creative risk,” Newman adds. “Good ones strike a chord with the audience and will be watched again and again. Whether it is Father Dougal learning about perspective or Del Boy falling through the bar, they are things that you will love and cherish and be among your favourite TV moments of all time.”

Get it wrong, however, and Newman thinks you’d better leave town. “People have a low tolerance of sitcoms that don’t make them laugh in a way that people won’t be personally offended if you make a bad property show.

“There is also the trifling matter of financial roulette — a Channel 4 sitcom might cost £300,000 an episode, nearly double the cost of a panel show.”

An indication of the enduring power of sitcoms is that only the other week someone in the street shouted “Dan!” at Mangan, even though this was the name of a minor character he played in I’m Alan Partridge half a decade ago. Sitcoms clearly get under people’s skin, even under the skin of the people in them, such as Mangan. “I got married earlier this year, and rather than naming the tables 1, 2, 3, 4, we named them after sitcoms. The top table was Till Death Us Do Part, others were Keeping Up Appearances, George & Mildred . . .”

It is time for another take. Mangan returns to the table, where a fake waiter pours another fake champagne. Creating the perfect sitcom? It’s enough to drive you to drink. Well, fizzy lemonade.

Comedy Showcase starts on Channel 4 on Friday 5 October, 2007, at 10pm.

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