Sheena Hastings: Can these leaders ignite the campaign? That's debatable

The Labour candidate for Leeds North East was not for turning, nor raising his gaze to those around him – possibly in case he got caught up in an unrehearsed moment or was laid siege by voters with questions like my own – on MPs' expenses, university tuition fees and care of the elderly.

Such moments can lead to out-of-control, headline-grabbing events like verbal handbaggings by dissatisfied citizens. They can also light up a dull, safe election campaign, and this one certainly needs a breath of oxygen to make it spring off the trolley.

Maybe tonight's first 90-minute television debate between the three main party leaders will light the fuse. The audience is expected to be big (at least for this one) and the risk factor for all three politicians is high – probably more so for David Cameron because his party is ahead in the polls. Should he fluff or flannel, or appear too glib, glossy or fake then a lot could be lost. The man with the least to lose is Nick Clegg, who will get to imprint his face and ideas on more people's minds – vital when great swathes of the electorate still struggle to put a name to his photograph. He will benefit from being treated as an equal and could blossom in the studio lights, knowing that his party could hold the balance of power come May. Gordon Brown will probably settle for not coming across as too leaden and perhaps his experience will gain him a point or two.

The debates are happening after six months of secret negotiation

between the parties and the media and the hammering out of a Stalinesque 76-point agreement covering every cough, sniff and camera angle and a stipulation that the studio audience must listen in silence.

All the leaders have undertaken US-style debate "combat training", and both Labour and the Tories have employed coaching firms involved in the Obama presidential campaign. Alastair Campbell has been posing as David Cameron for practice duels with Brown; Tory education spokesman Michael Gove has been acting as Brown in rehearsals with Cameron.

They can practise all they like, but there is always the glorious hope that a telling gaffe will erupt; after all, this is part of the reason many will watch – as is the longing that someone will depart from the prepared script and dare to sound authentic. Pseudo-improvised deadly one-liners will have been provided with the kit.

In the US, where the live screening of leaders' campaign debates began 50 years ago with the infamous encounter between John F Kennedy and Richard Nixon, many have taken an early fall. In that jousting tournament, Nixon looked sickly (he was actually unwell), pallid and shifty as the camera caught his eyes sliding sideways in apparent panic as beads of sweat formed on his face.

In contrast, Kennedy was handsome, tanned, understatedly elegant, calm and in command. Interestingly, those who had followed the debate on the radio felt Nixon had won; viewers put Kennedy ahead. Lesson learned: the importance of body language cannot be overestimated.

Gerald Ford, however well he might have grasped the facts off-camera, lost his grip during a debate and declared that Eastern Europe was not under Soviet domination. George Bush Snr repeatedly looked at his watch, such was his desperation under fire.

Conservative PR wizard Lord Bell would not expose Margaret Thatcher to television debates because he knew her driving, hectoring style would come across badly. Tony Blair didn't need to take debate proposals seriously while his popularity was high. With the race so tight, Brown now sees the virtue in risk.

"The purpose of the exercise is to get undecided voters talking about it the next day," says Stephen Coleman, professor of political communication at Leeds University. "To engage floating voters, they have to have both substance and presentation, coming across as having had experiences people will identify with. They have to seem ordinary but at the same time extraordinary. No-one would actually vote for the bloke next door as Prime Minister.

"One thing's for sure: we haven't got an Obama in this race. We're looking at the old political class trying to do something new."