`Chicago', the musical, tells of two women who prove that, sometimes, crime really does pay. Chicago's Cook County Jail, where the show is set, tells an altogether seedier story to Nigel Planer (right), one of its stars.

"Everybody here gets what's coming to him. It might be a bar of soap, or a tube of toothpaste, or it might be a beating." O'Hare, head of Special Operations Response Team (SORT), grins cheerfully as he is frisked at the entrance to the women's wing. Then it is my turn. O'Hare wears a dark green shiny suit, his belly hanging over his belted trousers, and has eczema all down the back of his neck. "Things ain't so bad now, but a couple of years ago I was called in every couple of days," he says.

The SORT team are in charge of "externals", which means gates, walls, entrances and exits, and any kind of unrest at the Cook County Jail, which houses approximately 10,000 inmates awaiting trial, sometimes for up to four years. Their average length of stay is 18 months.

So what am I doing here and why am I about to enter the women's block? Because Cook County Jail is the setting for Chicago, the show in which I am about to open in London's West End. Based upon a true case from the 1920s, it tells the story of two murderesses who, by manipulating the press and hiring the right schmoozy lawyer, manage to get themselves acquitted, turning into media stars in the process.

I play the dumb husband of one of the killers, who stumps up the money for the lawyer, thus inadvertently enabling his wife's transformation from cold-blooded murderer to front-page celebrity. It is a very modern tale which did unbelievably well on Broadway earlier this year in the wake of the OJ trials, and may well carry extra resonances in the West End in the light of recent courtroom events in Cambridge, Massachusetts.

Lest you get the wrong idea, I realise that visiting Cook County, Illinois, is possibly taking the actor research thing a bit too far; it's a comedy musical, for goodness' sake, and I am not Dustin Hoffman. But I happened to be in Chicago, and one call to the sheriff's office explaining my position seemed to have been all that was required to provide me with an escort for the day in the person of deputy sheriff Maric, lunch in an Italian restaurant frequented only by cops, and a guided tour around all 11 blocks of Cook County, including the murderers' wing, the court house, the local boot camp, the sheriff's office, the Michael Jordan Stadium and most of the other sights of Chicago.

Being typically English, I am somewhat embarrassed by this extraordinary hospitality, but it seems that they are used to showing showbiz people around and take my interest more seriously than perhaps I did myself.

But I suspect Americans take musical theatre more seriously than we do too. For them, a musical is not necessarily a piece of light entertainment, it is a major part of their cultural history, a respected art form, and a forum for social comment. In Kiss of the Spider Woman - the last show by Kander and Ebb, the authors of Chicago - the strident chorus of revolutionaries held up photographs of actual people who had "disappeared" in Latin America; their most famous show, Cabaret - despite being about Liza Minnelli in black stockings, as we all know - is set in the Berlin of the 1930s. Anyone who thinks that the Americans have no sense of irony should come and see Chicago.

The original 1924 building of the Cook County Jail now houses the men awaiting trial for serious offences, "not mere burglarising", as Maric puts it. And, possibly because of the age of the building, the conditions in there look pretty poor to me.

The narrow corridor winds around the edge of the block past the antique bars of the cells where the prisoners are housed. Two inmates are confined to an area about the size of my kitchen when I was a student. In each cell are the beds, laid out head to foot, a basin, a toilet, and no further room for a cat to hand-jive, let alone swing. Books, toiletries, toilet paper and other possessions are crammed between the bars. There's certainly nowhere else to put them.

Many of the guys are sleeping; 14 hours a day is the average, I'm told. One guy is doing push-ups - a new inmate, evidently - but most just sit, gazing into space. They do not acknowledge our presence, nor the conversation that Maric and O'Hare are having about them for my benefit. Contrary to my woolly liberal expectations, only a little more than 50 per cent are black. Suddenly, one Sean Penn lookalike becomes aggressive and starts to rattle his bars as we pass: "Sir? I got a complaint... Mr O'Hare, sir. The door is registering `open' when it is still locked... it's been like this for three days... you gotta do something about this, Mr O'Hare, sir..." It's this outburst that prompted O'Hare's remarks about toothpaste and beatings.

The events of the day begin to seem like an episode of Homicide or NYPD Blue. The only thing missing is the unsteady camera work and wobbly handheld close-ups. Large shirt-sleeved guys with guns called Valasco or Lipitschy or Zaralech have their feet up on desks while they munch on donuts; people do call deputy Maric "chief", and he does playfully punch them on the shoulder. Uniformed women cops, looking like Oprah Winfrey at every stage of her weight loss/gain, do bring in sad-looking hookers, while other female plain-clothes 'tecs drift past in T-shirts, jeans and massive gun- belts. Everyone is packing heat, from the telephone receptionist to the doorman. I must have been frisked and security X-rayed enough times in one day to win a Howard Marks achievement award.

The behaviour in the women's block is completely different to that in the men's. After we have been security-checked once more, we stand in the central corridor while Maric explains to me some finer point about the provision of the 30,000 meals a day required at Cook County. On seeing us, the women crowd themselves to the glass walls that separate us from their TV common rooms and signal to us. "Hey! Come here!"; "Hey, boy! I'm talking to you. You wanna blow job?"

Maybe we've overtaken the bromide trolley on its way from the male wing. But Maric explains, "Tragically, these women see the arrival of three men as their only chance of gaining access to some kind of power." Maric turns out to be quite the little feminist.

On the way out, he shows me three large boxes full of knives, mace, blades and general nastiness. "This is just what we picked up in the last 24 hours," he sighs.

As we prepare to leave, I have to wait while Maric and O'Hare discuss next Monday's tasks. Witnesses for a Mafia hit-man trial will have to be brought in secretly. "Last time this guy went in front of a jury, one judge committed suicide and the next one went to jail for accepting bribes," Maric quips. As a joke, O'Hare offers to lend Maric his own bullet-proof vest, and then talks to him about retirement. "I mean, maybe it's time, you know. I still got both my feet." This is a different world.

As Maric drives me back towards central Chicago, past block after block surounded by razor-wire fences, I feel more than ready to tackle a day's shooting opposite Tim Roth, with Harvey Keitel thrown in.

To quote a line from the show, "In this town, murder is a form of entertainment."

`Chicago' opens tonight at the Adelphi Theatre, London WC2. Booking (0171- 344 0055) to April