In January 1998, Ian Paisley announced that he was exiling himself from the Good Friday Agreement peace talks and would instead travel to Cameroon to preach to the sinners. I wrote to ask him if I could tag along and make a documentary. He agreed.

On the plane to Cameroon. I smiled across the aisle at him but he just sat there, quietly reading the Bible. I could see that he’d made detailed notes in the margins, in tiny, neat handwriting. This was impressive to me – Bible-wise, he definitely walked the walk. He later told me he could recite the Old Testament in English and in Hebrew.

Feeling the urge to break the ice I cleared my throat and leaned across the aisle.

"I imagine the Cameroonian singing style must be very different to the Northern Ireland one," I said.

He looked at me. “In what way?” he boomed.

“Well,” I continued. “Take the vocal harmonies . . . I assume they’re very . . . um . . .”

I trailed off. Perplexed, I could see that he was now furious with me. A steely silence ensued that continued for the rest of the flight. It continued through disembarkation, customs and passport control and into the courtesy bus, where it finally and mercifully ended.

“We Ulstermen are outgoing people,” he suddenly announced as the bus bounced through the potholed suburbs of Yaoundé. “We’re hospitable people. We’re good singers. These are characteristics that are sadly lacking in some Englishmen!”

The visit lasted eight days, during which time we travelled across country in two hire-jeeps, while Dr Paisley preached the gospel thrice daily to the locals. We had walkie-talkies to ensure nobody got lost on the long drives through the countryside. On our first trip my walkie-talkie crackled into life.

"Germany calling!" Dr Paisley hollered. "Germany calling! There's a good degree of racism in us all! Did you hear that? Racism in us all! Ha ha! Over!"

I was quite astonished. I wanted to make sure we’d captured the moment on camera. So I said, “I got a bit of a crackle there, Dr Paisley. Could you repeat that please?”

“You got no crackle at all,” he yelled. “You got the message loud and clear. Do not tell lies. Over and definitely out!”

Three hours later we pulled up at a small stone church in a forest clearing. Dr Paisley wandered over. He looked like he felt a little guilty about the walkie-talkie incident. He clapped his hands together. “Right. Time to work up a good pulpit sweat.”

From then on, Dr Paisley had three nicknames for me: The Jew, My Jewish Friend, and My Circumcised Friend.

It was dusk. Fifty or so locals were gathered on the grass. The church lights weren’t working, so Dr Paisley retrieved from his pocket a torch he’d bought at Heathrow. He propped it on the ground and switched it on. It cast a giant Ian Paisley shadow across the church wall, like Godzilla. And he began to preach.

And it was mesmerising. He stared out into the darkness, oblivious to the insects that were crashing into his face, attracted by the torchlight.

Years later, people would ask me if I was surprised by his incredible transformation into Martin McGuinness’s friend and ambassador for peace and compromise. Had I noticed anything about him in Cameroon that indicated such a conversion might occur?

I wish I could say I had, but there was nothing. In fact, on our last day together – at his mission house in the countryside outside Yaoundé – he sat me down and said, "Somebody once said to me, 'If it wasn't for your religious views you'd have taken Northern Ireland by storm'. But I won't give in. I am not sacrificing my religious views for anyone. I am a captive to the Bible."

I don’t know how Dr Paisley found a way through this fundamentalism to become the person he eventually became, but it’s wonderful that he did.

The thing that stays with me the most happened after Cameroon. I spent six weeks editing the documentary. There were moments I knew he’d like – the powerful torchlight preaching, the utter dedication to the Bible, the long, long hours travelling in arduous circumstances to preach the world of God.

But there was stuff in there that I knew he’d hate – the bullying, the casual anti-Semitism, the startling mood-swings. He was forever chiding me and also his local translator, Joseph.

The documentary's producer David Malone had offered to show Dr Paisley a cut of the film before it aired, to check it for factual accuracy. This always worries me, especially with someone as volatile as Dr Paisley, because "checking for factual accuracy" can easily turn into a scuffle for editorial control.

Dr Paisley, his son and the Rev David McIlveen, who had been in Cameroon and was at the time the head of the Save Ulster From Sodomy campaign, all showed up, looking very sombre and suspicious.

For the next hour Dr Paisley roared with laughter, slapping his knees, elbowing David McIlveen in the ribs. When it was over, he said, “Marvellous.”

“But dad,” Ian Paisley Jr said. “What about, um, ‘Germany Calling’?”

“Nobody will watch if it’s all preaching!” Dr Paisley said. “There has to be humour! Or else nobody will watch!”

There was no hubris to him then, no self-consciousness. That admirable lack of vanity has stayed with me ever since.

"Don't change a thing!" he roared. Jon Ronson is a journalist, documentary filmmaker, radio presenter and nonfiction author, whose works include The Men Who Stare at Goats