the magners glasgow international comedy festival is coming to town. But what makes a stand-up risk breaking his funny bones night after night? BY GRAEME VIRTUE

A STAND-UP comedian can be many things: a clown, a fool, a misanthrope, a space cadet, a machine-gun, a battering ram, even a woman on occasion. But most of all, they have to be a barometer. Walking into that spotlight, they must gauge the pressure in the room, trying to intuit the atmosphere even as they launch into their first routine, carefully analysing the - dread word - "vibe".

Those who make the most accurate forecasts thrive, tailoring their material to the audience like a skilled seamstress. Those who misread the signs fare much worse; their spiel unspools, their patter scatters and their shtick becomes stuck. In the jargon, they "die" on stage, and while there is often a shining bright light, it's difficult to float peacefully towards it if the disgruntled audience are in the way. Live comedy means making split-second judgements and dealing with the consequences in real-time - like Jack Bauer but with more knob gags. That's what makes it so exciting.

Yet the old-fashioned form of a man (or woman) with a mic cracking jokes is still thriving, despite cable TV channels stuffing their schedules with comedy panel-show repeats and the internet becoming increasingly clogged with wacky YouTube clips. So how does stand-up still manage to stand out in a world of information overload?

"Live comedy is a one-off experience that you can't replicate," says Des Clarke, one of Scotland's fleetest young comics. "People think that stand-ups do monologues, but it's genuinely a dialogue between the performer and the audience."

"It's the difference between eating a meal and watching someone eat a meal," adds Tommy Sheppard, who runs the Stand comedy clubs in Edinburgh and Glasgow. "On TV or the internet, you're only using two of your senses. With live comedy, you can be using all five, especially if you're sitting in the front row."

Sheppard is also director of the talent agency behind the Magners Glasgow International Comedy Festival, which will stage 350 live shows (including a gig by Clarke) over 18 days. With 99,000 tickets for sale, that's the biggest comedy festival in Europe - or, as some of the acts probably think of it, a hell of a lot of potential hecklers.

The festival may be funnelling in talent from all over the UK and beyond, but flicking through the brochure, the most recognisable names are invariably associated with TV. Some - such as French and Saunders - have been on the box for so long, it's difficult to imagine what they might be like in a purely stand-up setting (Royal Variety Performances don't count). There are other comics who have made the leap to telly seem so effortless, you wonder if they're trying to make a point by returning to perform among the great unwashed. (Talking of Jimmy Carr, has anyone considered he might be on TV all the time simply because his Mr Strong-shaped head is such a perfect fit for the camera frame?) The programme also contains a whole gaggle - giggle? - of comedians who seem equally at home in front of the cameras as they are on stage, including Clarke, Frankie Boyle (pictured on previous page), Dara O'Briain (pictured below), Karen Dunbar and Russell Howard. But if being invited on Have I Got News For You is a valuable staging post in a stand-up's career, such widespread exposure can sometimes be a hazard when they return to the clubs and theatres.

"Frankie Boyle is an example of someone who works really hard on his material, sharpening his lines to get the maximum effect," says Sheppard, "so he's very careful not to use the same stuff when he's on tour. One of the worst things that can happen to a comic is if someone in the audience has heard the joke before on TV and shouts out the punchline."

A joke is never as funny the second time you hear it, so repetition must be a real hazard when your best gags are being fired around the web. After minting a worldwide cult following with his frazzled online videos and expletive-soaked timewasting games, Limmy (Glasgow-based comedian Brian Limond, who's interviewed over the page) is back at the comedy festival performing live for his second year. "Limmy is someone people discovered online first who is now doing gigs," says Clarke. "The internet helped him find an audience - and the people who come along already know what he's about, which can be a real help."

So why are both TV veterans and web newbies jostling for the mic to perform live? Blockbuster stand-up tours can certainly be lucrative, but it's hard to think of another profession where a hard-earned reputation is gambled every night in front of a room full of strangers. No matter how many awards you might have won or panel shows you've chaired or DVDs you've sold, it can all evaporate in the time it takes for a misjudged gag to clunk to the floor.

Many of the metaphors associated with performing stand-up tend to be drawn from the circus: it's either tightrope-walking or high-wire acts with no safety net. For the comic, it probably seems most like sticking your head in a lion's mouth (when you haven't had the benefit of spending months training the lion). How does it feel just before a comedian takes the stage?

"It's like that feeling in the pit of your stomach when you're a wee guy in the back of the car and you go over a bump," says Clarke. "You're not sure if you like it or not, and it messes with your mind."

One of Clarke's first big breaks was the So You Think You're Funny? competition in 2000, and the heats were particularly formative experiences for him. "I was really nervous and I hadn't slept or eaten and I had all my notes on crumpled bits of paper," he says. "When I went on stage, it felt like I started to say everything at once, but it actually went well. I realised the audience were responding to me as much as the material. They found this tangential, self-interruption funny, and it was incredibly freeing. That was the foundation for a lot of what I'm doing now. Those audiences helped me find my voice."

Just as audiences expect to be entertained by a comedian, so there is another side to the transaction; the comic needs something from us. Call it approval, respect, or maybe even just some sympathetic guffaws. Often, of course, they get something less welcome: a heckle.

"Even the word has negative connotations," says Clarke. "Sometimes it's just someone in the audience joining in and all it needs is a wee putdown and you can keep going. But if you're too heavy-handed - I once heard a comic say I f***ed your mum' to a heckler - it can encourage the audience to take the heckler's side, and that's what will turn a gig."

Some performers can carve themselves a reputation for being a bit rough-and-tumble with their audiences (Brendon Burns and Jim Jeffries - both appearing at the festival - don't shirk from being combative). But it takes a very special kind of comic to judo-throw stunned, horrified silence into something approaching performance art. Which is why the "vibe" at Jerry Sadowitz's gig on March 21 will probably be fairly charged.

People think Javier Bardem was pretty scary with a compressed-air cattle gun in No Country For Old Men, but Sadowitz (pictured left) has been cheerfully slaughtering sacred cows in far more offensive ways for years. Walkouts are not uncommon; audience members offer removal instead of approval. He might be the exception that proves the rule. "Sadowitz is brilliant," says Clarke. "But he's one of the few comedians who can get away with heckling the audience."