Monday at the Manhattan County Courthouse begins with hundreds of people in a room watching a short film that opens with a vox pop of New Yorkers complaining about jury service. Then, a bit of history: the Greeks invented trial by jury, the Romans abolished it. In the Middle Ages, there was trial by ordeal: a trussed unfortunate is dragged to a lake and thrown in. The film ends with various people praising the American judicial system: a local judge, a TV anchorwoman; the resentful taxpayers from the opening scene have changed their minds.

Along with some 60 others I am called to a courtroom. Over the course of several hours of general questioning most people are dismissed and more are called. By the time 14 of us have been selected – 12 jurors and two reserves – we no longer look like a representative sample of Manhattan residents; we are whiter, for one thing. During the ‘voir dire’ process I am the first to be interviewed by the judge, answering various questions about where I live, what I do for a living, where I went to school. Not expecting her to ask me what I do for fun, I can’t think what to say. Other people say reading, travel, theatre. One man says that he works out with his personal trainer and takes care of the family estate in Connecticut; he does not end up on the jury.

The judge tells us to forget everything we have seen on TV or in the movies: this will be nothing like Law and Order or CSI. She is very like a judge on Law and Order, with her peremptory manner and New York accent. The main difference between this trial and all those I have seen on TV is the banality of the case.

The defendant is a young man with extremely bad luck. Approached on the street by a plainclothes officer who thought he looked like a man on a wanted poster, he ran off, straight into a second policeman. He was taken into custody and found to be in possession of twelve small bags of cocaine and $434 in bills of various denominations. On Tuesday we hear testimony from three police officers and two narcotics experts. Like an episode of Law and Order the trial is filled with red herrings. Much is made of whether the defendant had a wallet or not, whether the bags of cocaine were found in one pocket or two, whether it was cocaine or crack, whether he resembled the man on the wanted poster (he did). The defendant does not testify. On Wednesday we hear closing statements from both sides. The defence counsel talks a lot about Robert Downey Jr.

On Friday deliberations begin with an awkward pause. None of us has any experience of criminal trials beyond what we’ve seen on TV. We start with the first count: wilful possession of more than 500 milligrammes of an illegal narcotic substance. Not even the defence has contested the fact that the defendant had almost three times that amount on him; everyone agrees he is guilty on this charge. The more serious charge is ‘intent to sell’. We go around the room; 11 people offer hesitant variations of ‘I think he’s probably guilty.’ The forewoman asks why. We go over what we’ve heard, call for the court officer to ask to examine the evidence. When we go around the room again a woman who had originally said she thought he was guilty changes her mind. Shortly after this the forewoman changes her mind too, and decides he’s probably guilty. We spend four hours deliberating (lunch is brought to us), the role of solitary hold-out played successively by different people.

Someone suggests drawing up lists of factors indicating guilt and innocence, but no one can find any exculpating factors. A young man says that maybe there’s another explanation, and concocts an elaborate story about how the cocaine was all for the defendant’s personal use, and the reason he had all that (possibly stolen) cash on him was that he’d been wanting to buy even more but his dealer only had twelve small bags. No one else finds this remotely plausible. ‘Reasonable doubt,’ the young man says. We decide we need another explanation of reasonable doubt. We are led back into the courtroom where the judge tells us again what ‘beyond a reasonable doubt’ means. Back in the deliberation room it takes less than five minutes to reach unanimity. When the guilty verdict is announced the defendant smiles; in the public benches at the back of the courtroom two women bury their heads in their hands.