The cast members of BBC4's The Quatermass Experiment are sitting round a table in a central London church-hall. Wearing tracksuits, jeans and (for the men at least) stubbly chins, they are a picture of casual relaxation. As they pick at their lunchtime sandwiches, they bat nonchalant banter back and forth across the table.

The cast members of BBC4's The Quatermass Experiment are sitting round a table in a central London church-hall. Wearing tracksuits, jeans and (for the men at least) stubbly chins, they are a picture of casual relaxation. As they pick at their lunchtime sandwiches, they bat nonchalant banter back and forth across the table.

David Tennant, who has just starred in the title role of BBC3's Casanova and who is playing Dr Briscoe, reveals that his character has to deal with Victor Carroon (Andy Tiernan), an astronaut who crash-lands on Earth and starts to metamorphose into a bizarre, spiky, plant-like alien. "When Carroon becomes a cactus, it is rather hard to interact with him," says Tennant. He points to a cactus plant on a nearby props table. "There he is, deep in character already."

The actors even manage to raise a laugh during a script-reading, after a blood-curdling scream from The League of Gentlemen's Mark Gatiss (who portrays Quatermass's assistant Paterson) has just about made us all jump out of our skins.

You can guarantee, however, that the backstage ambience will be rather less jovial as the clock ticks towards 8.20pm this Saturday, 2 April. For that is the hour at which The Quatermass Experiment will be broadcast - the first live, one-off drama on British television for more than 20 years.

Gatiss contemplates the sheer scariness of going live with a complicated, two-hour sci-fi drama. "I'm sure there are a huge number of actors who said, 'Live? You must be kidding!' before they came to us." Tennant adds: "I had no hesitation about accepting this job. I thought, 'Why the hell not? It'll be a hoot.' But as the big day approaches, you begin to realise what this actually entails and the sphincter noticeably tightens. What we all fear most is that total brain shut-down, where you don't know who you are, where you are or why you are there. At one point, I have to talk about 'post-catabolic residues'. I've no idea what it means, but I've been told it's something to do with remnants of dead-cell tissue."

The cast members are also terrified of "corpsing" (breaking into uncontrollable giggles). "We corpsed this morning," Tennant says, "and once you start, it's like a runaway train. Afterwards, I went into a cold sweat, thinking, 'What if that happens on the night?'" Gatiss chips in. "If it does, BBC4 will immediately cut away to the potter's wheel. They'll have one standing by in the next studio."

Jason Flemyng, best known for his work with Guy Ritchie on Lock, Stock and Two Smoking Barrels and Snatch, plays the eponymous hero, Professor Bernard Quatermass. In Nigel Kneale's durable sci-fi drama, which first went out in 1953, the professor faces a race to save the world when Carroon returns from space contaminated with a lethal infection. "I'm flattered to be asked to play this role," says Flemyng. "There is no actor on earth who wouldn't want to utter the line, 'Listen to me, I'm Quatermass.' That's one of the boxes you really want to tick as an actor."

All the same, Flemyng is well aware of the potential pitfalls. "Everyone involved in this is excited by the fact that it's live," says the actor, who has starred in 49 feature films, "but it's not without risks. It could look like those dramas they used to concoct on The Generation Game. We're not allowed to have cards with the lines secreted around the set, but there might still be a certain amount of writing phrases such as 'voracious rapacity' on our shoes."

But Flemyng reckons that may be one of the main attractions. "You know how we all watch Formula One for the crashes? There will be a certain element of that among the audience for The Quatermass Experiment. Members of Equity, in particular, will be licking their lips - 'You think you're so clever - well, prove it!' This is the Equity equivalent of Formula One - without the budget."

The production will have the run of a vast, disused RAF base in Surrey. The logistics of a live, two-hour drama are considerable. In one sequence, the camera will cut away for just 10 seconds from a laboratory to a press conference. When it returns, the lab must be totally destroyed. "They're going to use golf-carts to transport us around the set because we have to dash between scenes so quickly," Tennant reveals. "We're also all being assigned a runner each because they don't trust actors. 'Next up is that scene with the test tube?' 'Which one?'" Gatiss jokes: "It could still be like that scene in This is Spinal Tap where the band are wandering around lost in the backstage labyrinth. Either that, or the camera will catch us unawares behind a door telling each other theatrical anecdotes - 'And then I said to Bernard Delfont...'"

The producers of The Quatermass Experiment, which is the centrepiece of TV On Trial, BBC4's week-long season recalling the televisual highs and lows of the past 50 years, are equally excited.

Sam Miller, who has helmed both television shows (This Life) and feature films (Among Giants), is relishing the challenge of real-time TV. "There is nothing remotely as exciting as this. This is not a project for the faint-hearted, but at the same time the live element here should not be like tying one hand behind our backs. We must harness the actors' live energy, so that viewers notice a 10 per cent difference from normal, pre-recorded performances."

Kneale's work lends itself to live broadcast. Gatiss, the author of four Doctor Who novels and an episode in the current series of the Time Lord's adventures, recalls that "Quatermass was the first massively popular TV series. In the early 1950s, it landed like a rocket in the middle of the schedules, which in the years after the war were a wasteland. Those wonderful stories about the pubs all emptying for the weekly episodes of Quatermass are true; 1953 was the year of the coronation, the conquest of Everest and Roger Bannister breaking the four-minute mile - and Quatermass chimed with that sense of excitement about a new era." Kneale, now in his eighties, is a consultant on this production.

Miller's view is that this project is one of those things that only TV can do. "Think about the sense of togetherness the nation gets from watching a big football match live on TV," he says. "If you could whip up the same feeling with a drama, it would be sensational. As the TV channels proliferate, we're losing that sense of a shared experience, that sense of, 'Where were you when this happened?' I hope this drama could foster that. I'd love this to be like an England vs Germany penalty shoot-out in a World Cup semi-final - only this time, we win!"

'The Quatermass Experiment', 8.20pm, Saturday, as part of BBC4's 'TV on Trial', a season reflecting on TV since the 1950s, in which viewers are asked to vote for their favourite decade. Further details on

Source: The Independent 31st March 2005