We were working undercover and posing as tourists so we had to think through the implications of that for the people we were working with.
We produced security protocols and risk assessment documents and researched each scenario we might find ourselves in based on the leads we were following, looking at the environments we might end up in and how we might tackle any risks. The major risks were being caught by the police or an informer spotting us and alerting the police.
We'd take a situation such as meeting someone in an apartment block in the city and ask: Is it a safe thing to do? How would we get there? What are the risks when there? What would our cover story be in the event of somebody arriving at the door?
We drafted a security procedure which included a protocol that we would text back to a central command on a daily basis. They would then text the UK the next day so they knew the whole team was safe. Central command would wait for the text:â€ˆif it didn't come within a set period they would text us back. If they got no response they would alert the UK.
If you are trying to avoid being detected when you are filming, you need to plan for all eventualities from the outset.
It was a long process gathering people to interview and discuss the programme with. We employed people who were removed from us by one. I would know someone and then they would tell me about someone else who was safe.
We learned there was a group for families whose children had disappeared. We had two main leads: we knew of the parents' group and we knew of the detective who was searching for missing children. We knew the lives of the families we spoke to would be made difficult if they were found out; they'd be monitored and followed and could face detention. We worked on ways of minimising that risk before we went to China.
We are in contact with the families [since the programme aired] to make sure they are safe. If the Chinese authorities do come down on them for speaking out, and they needed human rights lawyers, we will be able to help them.
The leader of the parents' group told us his phone was being monitored. We changed our SIM cards for each contact to prevent cross-contamination. If one card becomes contaminated by monitoring, all they will ever get is contact between us and that one person.
When we filmed the rescue attempt of a trafficked girl from a brothel by the detective, it was in a town that foreigners never go to.
Neither I or the associate producer could go in, so we asked the detective to secretly film it. I set up in an hotel, from where I directed. The associate producer accompanied the detective to a hotel opposite the brothel, where she stayed with the mother of the girl. We communicated via text message.
The biggest fear for us was that if the rescue went wrong, the detective would be forced to call the police and we would have been found out. That would have been the end of the project.