The Coltrane sideshow

Cracker didn't pick up 28 significant television awards on its journey into the nation's heart by stinting on the detail. All those little bits add up: the close-ups of Robbie Coltrane, cigarette smoke curling around his brow as he posits his theories at the scene of the crime; the way the camera lingers fractionally longer than is comfortable over an interrogation or a moment of violence; and the way that, during the filming of an episode in the new series which begins tomorrow, Barney the body extra spent two hours in make-up having prosthetic wounds attached to his features to give him the appearance of someone who has been savagely murdered with a broken beer bottle. And this was even though he was going to be filmed exclusively face down in mud.

"I'm well used to it," said Barney as he gingerly re-attached a wound to his slashed face during a break in filming. "I done GBH2, a Manchester gangster movie, and I was shot in the eye in that. You know, blood and that everywhere. And I was a thug in a video for 808 State, the band. I usually play a thug or a body. Not that I am, like, a thug."

Barney was one of a dozen extras called up by the producers of Cracker one day in the summer to fill in the few gaps on screen that Robbie Coltrane's Fitz doesn't occupy. There were insurance clerks playing WPCs, the bloke who normally acts as a guide at Manchester's Granada Studios Tour kitted out as a bobby and, playing a forensic photographer snapping Barney's final resting place while dressed in a paper boilersuit (bought - this is Cracker accuracy - from the outfitters that supply the Met), and there was me.

A film set is probably the biggest waster of productive time invented. There is always something going on, but it rarely involves more than two people at a time. Before any shooting takes place, lighting has to be adjusted, camera angles worked out, sound levels checked. And since the extra is at the bottom of the film-set food chain, it is the extra's time that hangs heaviest: your part in Cracker's success is, to be honest, thin.

Thus, on a wet and squally day in August (the programme is filmed in Manchester) I found myself spending a lot of time talking to police officers. Or rather to insurance clerks and guides from Granada Studios Tour who looked uncannily like the real thing.

"I've had all sorts when I've been dressed in this uniform," said Liz Roberts, who landed her non-speaking, non-acting role as a background bobby after applying to an advert in her local job centre.

"One day I was walking between two locations along the street, still in costume, and this car drew up just in front of me. And this bloke I was at school with stuck his head out of the window and shouted: 'My God, Liz Roberts is a pig.' "

Such is Cracker's thirst for accuracy that the same extrashave played the uniformed branch throughout the programme's life: even in the wilder reaches of Michael Howard's imagination, police manpower is limited. So it is only logical that the same faces should crop up in the background.

Like Robbie Coltrane and Geraldine Somerville, who plays Detective Sergeant Jane Penhaligon, therefore, Liz has appeared in all three previous series of the drama. You may not have noticed her, but after her performance knocking on a door during house-to-house inquiries in one episode, all her mates did.

"I walked into my local the next night," she remembered, "and the entire pub stood there and did a mime of knocking on the door." Moments like that presumably make it worth spending most of your holiday from your insurance company employer sitting on the bonnet of an ersatz panda car yawning while the first assistant director supervises the construction of a small railway system along which the camera will track. It certainly couldn't be the financial return that persuades people to dress up in paper suits or to frighten their old school chums by impersonating policewomen - pounds 45 for a very long day is the going rate (although Barney the body got more, thanks to an inconvenience allowance for all those prosthetics).

"No, I love it," said Liz Roberts. "I love just being a part of something everybody loves." Not that being an extra is easy. All that hanging around merely feeds the nerves, which boil in the stomach as you anticipate being responsible for some awful gaffe - dropping your props, or tripping and ripping your paper suit - a cock-up that will surely appear on a show fronted byDenis Nordern.

Thus, after two hours of uninterrupted hanging around, the moment the first assistant director called the proper actors on to the set - "positions, luvvies, please" (I kid you not) - it would not have been a surprise had the forensic photographer fainted on the spot.

It is fortunate, then, that Cracker involves Robbie Coltrane. The location for the shoot was behind a pub directly underneath platform 14 of Manchester Piccadilly station. The pub had been commandeered as a dressing room.

The first we lower orders were aware of Coltrane was when a commotion started outside the pub. "Yee faggin bashtudd," we could hear a huge Scots voice yelling. "Ah've been thrown out of better pubs than this, yee bashtudd. No one throws me out o' their pub and gezz away wi' it." As heads spun round from every point on the set (and up on platform 14), it quickly became clear this was Coltrane, impersonating a drunk being thrown out of a boozer. The country's favourite actor, it seems, is a man genetically incapable of being called from his dressing room without making a comedy performance out of it.

It didn't stop there. For the rest of the day, he gave a bravura turn for the benefit of the cast and crew. He constantly cracked jokes and told yarns, lifting Geraldine Somerville off her feet in a vast bear- hug one minute, shadow boxing with Ricky Tomlinson (Wise) the next and making the technicians wet their trousers in between.

Even the extras were privileged to a Coltrane shaggy dog story (involving a Scotsman, the SAS and an impromptu operation on the thumbs), a yarn so brilliantly told there was little energy left when it had finished to fuel nerves. With Coltrane around, the scene we had waited so long to film was soon over. It concerned him arriving at a murder site, checking the body ("not too much blood on Barney's head," cautioned the director, "remember the ITC guidelines"), presenting a theory to his unimpressed colleagues as to how it came to be there and then running off to attend to a plot detail which cannot be revealed here lest it ruin your enjoyment of the programme: roughly two minutes' action from most of a day hanging around.

You wondered, given the staccato manner in which they film the programme, how it achieves the tension, the continuity, the seamless verisimilitude that is its trademark. "I wonder that myself," said Charles McDougall, the director, as the crew enjoyed the biggest perk of film-set life - free gourmet catering after the shooting stops. "Have you enjoyed yourself by the way? Yeah? Well, you wouldn't if you had to spend another day hanging around here."

After that it was all over for two fake police officers, three pretend forensics, a couple of bogus undertakers and a body. As this unimpressive army made its way back to the make-up wagon, someone caught sight of Barney's fast-fading wounds. "That looks really, you know, yuck," she said. "You ought to take care, love."

'Cracker' is on Sundays,

ITV at 9pm