Yasiin Bey, formerly known as Mos Def, had agreed to submit his body and fame to a form of torture. The body bit would involve being chained to a feeding chair while doctors inserted more than a metre of rubber tubing up his nose, down his throat and into his stomach. His fame would hopefully draw attention to that fact that this is happening daily to 45 hunger strikers in Guantánamo Bay.

Reprieve has always been well aware that in its fight against human rights abuses there is no substitute for the court of public opinion. Clive Stafford Smith, the organisation's founder, is often the first to volunteer himself for some sort of extreme treatment but, in his words, "nobody has the faintest clue who I am". Which was why I had agreed to ask Bey.

We met at his hotel. David Morrissey, a patron of Reprieve, had brought in Asif Kapadia (the documentary maker behind Senna) to direct the film and together they discussed its style and aims. Meanwhile Kat Craig, legal director of Reprieve's Guantánamo team, had called on Dr Adeeb Husain to perform the medical procedure and talk Bey through what was going to happen the next day.

Having volunteered to fast that day, Bey was tired and by now it was late. Despite this, he was attentive. His only request to the doctors was that they manipulate nothing and reproduce the experience of the detainees as accurately as possible. He hung on every word as Craig related the testimonies of Reprieve's clients.

She read from the account of Samir Mokbel, who said to his lawyers: "I will never forget the first time they passed the feeding tube up my nose. I can't describe how painful it is to be force-fed this way. As it was thrust in, it made me feel like throwing up. I wanted to vomit, but I couldn't. There was agony in my chest, throat and stomach. I had never experienced such pain before. I would not wish this cruel punishment on anyone."

I collected Bey from his hotel at 10am the next day. His mood was upbeat, his focus firmly on what was about to happen. The stories he had read of Shaker Aamer, whose detainment came before the birth of his child, seemed to strike the strongest chord. He texted his own wife and sent his family love as we arrived at our destination.

There was no rehearsal: after all, no acting would be required. He swapped his black leather jacket, jeans and designer shoes for an orange jumpsuit. In an instant, he was no longer Mos Def – rapper and Hollywood star – but a powerless prisoner. Morrissey and I were to be the "guards". We shackled his wrists and legs. Then we chained them tighter and tighter, to be sure he couldn't get loose.

*Yasiin Bey about to be force fed to highlight the treatment of Guantánamo Bay prisoners.*

We were already oblivious to the cameras. The doctor lubricated the tubing and without warning began feeding it up Bey's nose. He squirmed and spluttered. A second doctor held Bey's face in place as he began to yell out. Morrissey adjusted his grip and locked his forearm around Bey's brow. The fight continued and grew in intensity. Bey freed one arm and writhed so hard the tubes fell out.

By now the doctors were shaking. But there was a job to be done. I pinned down the loose arm so the "detainee" couldn't interfere with the tubes while Morrissey pushed down on his chest in an attempt to give the doctors some space. They lubricated the tube once more and went in for a second attempt.

"Stop, stop, Asif, please stop!" Bey screamed. In that moment it was clear there was nothing "pretend" about what was happening. We were instantly shocked out of our assigned roles. "I can't do it, I'm so sorry," Bey spluttered. "I'm so sorry."

What had we done? Was this a stunt too far? He wept as Morrissey gently rubbed his back. "It's OK," he said, "it's over now." Still chained, this famous artist had become in an instant so small. He seemed overwhelmed by his own brief glimpse at the humiliation that his country inflicts on its prisoners.

As the room calmed, we smoked cigarettes and talked about the US's decision to hold these men, some of whom have been cleared for release. "How is continuing to hold them politically expedient?" he asked. "This isn't embarrassing?"
Safe in the car as we drove away, he turned the music up. "These people have no idea," he remarked as we sped through the West End of London, busy with shoppers, and we continued in silence until we reached his hotel.