In spring 2009, I began shooting what would become Just Do It: A Tale of Modern-day Outlaws, a film that set out to capture the world of climate activists as they plotted their way towards the Copenhagen talks in December.

And I was determined not to get any of them into more trouble than they would have on their own. So I accepted an invitation to a Channel 4 legal briefing. There was much warning about what you mustn't do when filming with thieves, drug addicts, con artists and so on. The message was: "Whatever you do, don't trust or help them. Remember – they are criminals." This didn't feel relevant. Sure, the people I was filming nearly all had criminal records for nonviolent direct action, but they hardly lacked a moral compass.

During the tea break, I put my situation to the lawyer. "Ah," he said. "They are still technically criminals, so all that I said would apply." Then, looking worried, he asked: "This isn't one of our projects, is it?" Not for the first time, I was glad I didn't have a TV commission for this. I had to be free to use my own judgment.

It all started when I was recruited to act as videographer on the 2008 Drax coal train action, in which protesters hijacked a coal train en route to one of Europe's largest power stations. I could see that a much more important film could be made by capturing the events before and after the actions, letting the audience get to know the people who were willing to risk their liberties, and even their lives, to protest. I believed in what they were doing and wanted to help get their message out.

When I began to enquire about the possibility of filming prior to an action, I got a resounding no. A not unfounded distrust in the media, combined with the risk of incrimination for conspiracy and incitement, did not make people warm to the idea. Then, on 1 April 2009, I took a couple of cameras down to the Climate Camp protest during the G20 meeting in London. Ours were the only cameras to get clear footage of the clearance of the camp at Bishopsgate. As a consequence, the camp legal team got in touch, asking if I could produce an edit to accompany a report.

The film got a lot of attention. I was pleased to be able to show how disproportionate the police violence had been. I realised how active the runup to Copenhagen was going to be. It's now or never, I thought, and threw myself into Just Do It. I did a lot of talking to a legal firm about the consequences for people in the film. To my surprise, they were supportive, helping me to work out a protocol.

When people break into a site, chain themselves to things, and wait for the authorities to arrive, they pretty much accept that they will be arrested, and that their participation will become a matter of public record. So I didn't have to worry about identities. The thing I had to avoid was allowing evidence of them conspiring (or inciting others to engage in criminal activity) to fall into the hands of the authorities.

We established that the safest thing was for the police not to know about, or gain access to my material. If I were arrested at an action, the police could search my house. This meant I was to keep no logs, no paper trail of footage, no tapes at my address. I would film a meeting the night before an action, and a runner would drop that tape at a safe house.

I then travelled with, and filmed, those activists most likely to be arrested. Just before the action, I would hand that footage to a runner, meaning I started an action with a clean tape. I'd film the first few moments – the cutting in, the locking on, the police arriving – and send that footage off, this time to be edited for the news and shipped out immediately. No tapes would be recovered until any resulting court cases were over.

There was another thing that made the lawyers wince. Normally a film-maker would get everyone involved to sign away their say as to how footage would be used. I didn't. These people were consenting to being filmed while doing something technically illegal: it was important to give them more say.

At the Channel 4 briefing, they warned against aiding and inciting. But when you see people attempting to fix the world using only their bare hands, D-locks, superglue and bunting, how do you not offer them a hand? If the state says that makes me a criminal, well, I say it's not really up to the state to decide. I am certain history will vindicate these people. Resisting what you think is wrong, and empowering yourself to change it, is always the right thing to do.