|Al Jazeera's Slavery: A 21st Century Evil examines slavery around the world and asks why, hundreds of years after it was abolished, it continues to exist [GALLO/GETTY]
"Tread softly," an interviewee once advised me, "because you step in our lives."
Stepping into the lives of men and women who until recently were slaves demands the softest of treads. The experience of being enslaved - forced to work for no pay, often under the threat of violence and with no hope of escape, burns deep and leaves wounds which take a long time to heal.
From impoverished and often illiterate Thai farmers to women forced into prostitution; from men tricked into servitude in Brazil's brutal charcoal industry to entire families trapped as bonded labourers in Pakistan's feudal brick kilns; we met and filmed dozens of slaves for this series.
The ethics of filming
How do we film these slaves? What duty of care do we have to these men and women whose lives have been scarred? Each interview would essentially mean them reliving their ordeal. Most wept uncontrollably as they told their stories. How do we step softly into and then out of their lives?
It is a question that should always haunt us as filmmakers. Doctors have a simple baseline from which to work with their patients: "First, do no harm." How do we balance the harm which we will, ineluctably, do with the very genuine public interest in telling these stories?
At the start of production we set three main criteria. The first was that we would work with voluntary organisations that were in contact with the slaves and former slaves, and that, except where this was genuinely not possible, we would ensure that they were present at the filming.
The reasons for this were both ethical and practical. The ethical dimension was that, almost by definition, our interviewees would have only a limited knowledge of the breadth and the power of broadcasting. A slave gets very little opportunity to watch television, much less to understand fully the impact that being filmed could have on their lives.
The practical reason was that each would need support after we turned the cameras off. The stark truth of this was brought home when we filmed a recently escaped sex slave in the Republic of Moldova.
Dorina was just 20 years old and had been tricked then trafficked into prostitution. She had been forced to service the sexual demands of up to seven men a day. Throughout the filming she wept quietly as she explained what she had endured. The moment we finished she stepped outside to smoke and collapsed in the street.
The second was that we would guarantee anonymity where either the interviewee asked for it or, and just as importantly, where we decided that full exposure might lead to reprisals.
In the opening film of the series, the Thai men who had escaped from slavery were free and unlikely to face any adverse reaction from their alleged former slave-masters. But their families back in northern Thailand were very definitely at risk. We ensured that no-one would be able to identify either the men or their relatives.
The third criterion was the easiest. Each of the men and women was eager to tell their story, desperate that someone somewhere would know how they suffered. As Kevin Bales, the world's leading expert on modern slavery, puts it: "Slaves have no voice." Our promise to them was that we would amplify their voices and do so globally.
Seeking out the slave masters
But there are two sides to the equation of modern slavery. Slaves are one side; those who exploit them - whether they sell, trick or force them into servitude - are the other. We were determined that this series would do more than highlight the existence of what is, unquestionably, a very modern evil. We felt it was vital to examine not just what slavery is today, but why it continues. And for that we needed the slave-masters.
There is rarely any mileage for someone accused of a heinous crime in submitting him or herself to the public scrutiny of a televised interview. How then to get them on screen, to put human faces to the roll call of slavers?
Where possible we took an open and direct approach. We spoke with them, explained carefully the nature and thrust of our series, and promised them they would be given space to state their cases. Rageh's interviewing technique - non-aggressive and calm - was a key factor in some of these men speaking on camera.
But in some cases this would not work. Men who are actively involved in the modern slave trade and profiting from the misery it inflicts have no incentive to be identified. For them we needed to resort to undercover filming and subterfuge.
In Moldova we used hidden cameras to capture local pimps offering to sell us women and explaining how they shipped their human cargo across borders and into the European Union. For our film about China's vast gulag of prison slaves, we set up our own dummy business and approached a Chinese company which openly boasted of its use of prison slave labour to produce goods, illegally, for export.
There are clear and stringent rules covering both undercover filming and 'sting' operations: neither is it undertaken lightly. For each we went through a careful and protracted process of research and approvals to justify the public interest involved.
Why does slavery persist?
Yet exposing the slave-masters and identifying what led them to sell another human being into slavery or to profit from keeping them in bondage still does not fully answer why hundreds of years after slavery was legally abolished, it persists.
To do that requires a forensic peeling back of the layers of global geopolitical policy. This sort of highly detailed examination does not lend itself to the format of a 25 minute film. Yet if the series was to bring real understanding and hopefully contribute to a process of change, it was surely vital.
For this reason we devised a different format for the final programme in the series: An open televised public debate with questions posed to a panel of those who direct or seek to influence government policies throughout the world.
As we near the conclusion of a year's work on this series, we have a pretty clear idea of what questions we want to pose. But just as important are the questions that you, as viewers, feel need asking. For that reason, we invite you to do just that via this website: send us your questions and we will ask them - publicly and on camera - of the panellists in this debate.
Slavery today exists in ignorance and darkness. We want to shine a light. With your help we can do so.
Tim Tate is the series producer of Slavery: A 21st Century Evil.
Slavery: A 21st Century Evil can be seen from Monday, October 10, at the following times GMT: Monday: 2230; Tuesday: 0930; Wednesday: 0330; Thursday: 1630.
Click here for more on the series.