In 1987 I was 20 and the student union entertainments officer for the Polytechnic of Central London. One day I was sitting in the office when the telephone rang. I picked it up.

"So Frank's playing tonight and our keyboard player can't make it and so we're going to have to cancel unless you know any keyboard players," said a frantic voice.

I cleared my throat. "I play keyboards," I said.

"Well you're in!" the man shouted.

"But I don't know any of your songs," I said.

"Wait a minute," the man said.

I heard muffled voices. He came back to the phone. "Can you play C, F and G?" he said.

The man on the phone said I should meet them at the soundcheck at 5pm. He added that his name was Mike, and Frank Sidebottom's real name was Chris. Then he hung up.

When I got to the bar it was empty except for a few men fiddling with equipment.

"Hello?" I called.

The men turned. I scrutinised their faces. In the three hours since the phone call I'd learned a little about Frank Sidebottom – how he wore a big, fake head and there was much speculation about his real identity. Some thought he might be the alter ego of a celebrity, possibly Midge Ure, the lead singer of Ultravox, who was known to be a big Frank Sidebottom fan. Which of these men might be Frank? If I looked closely would there be some kind of facial clue?

Then I became aware of another figure kneeling in the shadows, his back to me. He began to turn. I let out a gasp. Two huge eyes were staring at me, painted onto a great, imposing fake head, lips slightly parted as if mildly surprised. Why was he wearing the head when there was nobody there to see it except for his own band? Did he never take it off?

"Hello, Chris," I said. "I'm Jon."

Silence.

"Hello ... Chris?"

Nothing.

"Hello ... Frank?" I tried.

"HELLO!" he yelled.

Another of the men came bounding over to me. "You're Jon," he said. "I'm Mike Doherty. Thank you for standing in at such short notice."

"So," I said. "Maybe we could run through the songs? Or ... ?"

Frank's face stared at me.

"Frank?" Mike said.

"OH YES?"

"Can you teach Jon the songs?"

At this Frank raised his hands to his head and began to prise it off, turning slightly away, like he was shyly undressing. I thought I saw a flash of something under there, some contraption attached to his face.

"Hello, Jon," said the man underneath. He had a nice, ordinary face. He gave me a sheepish smile, as if to say he was sorry that I had to endure all the weirdness of the past few minutes but it was out of his hands.

Before I knew it we were onstage. As we played I watched it all – the band assiduously emulating the tinny pre-programmed sounds of a cheap, children's keyboard, the enraptured audience, and Frank, the eerie cartoon-character frontman, his facial expression immobile, his singing voice a high-pitched nasal twang.

After that night – the greatest of my life – a year passed. Life went back to normal. Then Mike phoned and asked if I wanted to be in Frank's band full time. So I quit college and moved to Manchester.

And there I was, in the passenger seat of a Transit van flying down the M6 in the middle of the night, squeezed between the door and Frank Sidebottom. Those were my happiest times – when Chris would mysteriously decide to just carry on being Frank. Nothing makes a young man feel more alive and on an adventure than speeding down a motorway at 2am next to a man wearing a big fake head. I'd watch him furtively as the lights made his cartoon face glow yellow and then black and then yellow again.

I am writing this 26 years later. The music journalist Mick Middles recently sent me his not-yet-published biography Frank Sidebottom: Out of His Head. His book captures perfectly that "rarest of journeys" when an onlooker got to see the man born Chris Sievey turn into Frank. "The moment the head is placed the change occurs. Not merely a change in attitude or outlook but a journey from one person to the other. I completely believe that Chris was born as two people." Middles likens Chris to transgender people, trapped in the wrong body.

I never understood why Chris sometimes kept Frank's head on for hours, even when it was only us in the van. Under the head Chris would wear a swimmer's nose clip. Chris would be Frank for such long periods the clip had deformed him slightly, flattened his nose out of shape. When he'd remove the peg after a long stint I'd see him wince in pain.

Frank's character was of a child in a northern town remaining assiduously immature in the face of adulthood. He was a paean to ordinariness. But Chris wasn't ordinary. He was chaotic. Sometimes, on the way back from some gig, I'd become aware that we were taking a detour to some house somewhere with some women we somehow met along the way. There would be partying while I sat outside on the sofa.

In the van I'd listen to Chris's stories, trying to understand him. He reminded me of George Bernard Shaw's unreasonable man: "The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man." Chris was the unreasonable man, except the world never did adapt to him and he never made any progress. Like when Frank was asked to support the boy band Bros at Wembley. There were 50,000 people in the crowd. This was a huge stage for Frank – his biggest ever, by about 49,500 people. It was his chance to break through to the mainstream. But instead he chose to perform a series of terrible Bros cover versions for five minutes and was bottled off. The show's promoter, Harvey Goldsmith, was glaring at him from the wings. Frank sauntered over to him and said, "I'm thinking of putting on a gig at the Timperley Labour Club. Do you have any tips?"

We crisscrossed Leeds and Bury and Sheffield and Liverpool playing the same venues over and over again. Time passed and the audiences grew to 750 and sometimes even 1,000. It was consequently baffling for me to become aware of a growing sense of discontent in the van. Chris had been asking friends to perform cameos between the songs on his records. In this spirit he had asked his brother-in-law's friend Caroline Aherne to voice the part of Frank's neighbour, Mrs Merton. Afterwards, Caroline decided to keep Mrs Merton going. She somehow got her own TV show, The Mrs Merton Show. She won a Bafta. Her followup series, The Royle Family, won about seven. The Royle Family Christmas specials attracted audiences of 12 million. And meanwhile we were crisscrossing Manchester and Bury and Leeds and Sheffield and Liverpool in our Transit van.

Frank Sidebottom with Jon Ronson in Birmingham in the 1980s Photograph: Jon Ronson

The band's guitarist Patrick Gallagher told Middles: "It wasn't Caroline's fault. Chris was totally out of control. Whereas, say, Caroline Aherne had a single vision and could just pursue that, Chris might have a fantastic idea, and then, just as the point where it might actually get somewhere, he would spin off onto something completely different. That's OK for a while but it tended to piss people off because they never knew where they stood."

Suddenly everyone around us was becoming famous. My next-door neighbour Mani had a band. They became The Stone Roses. Our driver, Chris Evans, left us to try and make it in radio. By 2000 he was earning £35m in a year, making him Britain's highest-paid entertainer.

There is always a moment failure begins. A single decision that starts everything lumbering down the wrong path, speeding up, careering wildly, before lurching to a terrible stop in a place where nobody is interested in hearing your songs any more.

With Frank I can pinpoint that moment exactly.

"Chris wants to have a rehearsal," Mike told me one day.

"Why would Chris want to rehearse?" I said.

"To take things up a level," Mike said.

Chris's house was in a normal, nice, modern cul-de-sac. His children were playing outside. His wife, Paula, answered the door and told me to go to the spare bedroom. I walked up, passing the bathroom and glanced in. Staring back at me from the sink was Frank's head.

"In here, Jon," I heard Chris shout.

I opened the bedroom door. And stopped. A man was standing there, maroon shirt tucked smartly into neat black jeans. As I walked in he started playing a tight soul-funk riff with seeming nonchalance, but I understood it to be an act of aggression.

"Who ... are you?" I said.

"I'm Richard," he said. "From the Desert Wolves."

I'd like to say that during the years since Richard the bass player took an instant dislike to me – a dislike that only intensified during the months that followed before the band imploded, and climaxed in him yelling that he'd like to break my "keyboard-playing fingers" – he went on to have a disappointing life. But he didn't. He became one of the world's most successful tour managers, looking after Woody Allen and the Spice Girls. He currently manages the Pixies.

Richard was not the only proper musician Chris brought in. A skilful guitarist and a saxophone player turned up in the spare bedroom too. We began to sound like an excellent 1980s wedding band.

Chris told me to book us the biggest tour we'd ever undertaken. He choreographed it so I would begin the show. I'd walk on stage, alone, into a spotlight, and play a powerful C with my left forefinger. The synth brass tone – the most stirring of all the Casio tones.

We hired a people-carrier instead of a Transit van and set off to our first venue. The mood was pumped. The old band members had a certain avant-garde loucheness. But this new band: I felt like I was in a college sports team. We soundchecked. The place was packed. And then I walked out into the spotlight. And in the space of that first song – our classic Born in Timperley (to the tune of Springsteen's Born in the USA) – the audience veered from fevered anticipation into hoping we were playing a weird joke on them into realising with regret that we were not. The NME savaged us. By the end of the tour we were playing to almost-empty houses.

Chris returned to Manchester to a court summons. He owed £30,000 in back taxes. On the day of his court appearance the judge told him it was a very serious matter and had he considered a payment plan?

"Would a pound a week suffice, m'lud?" he asked.

"No it would not!" the judge shouted.

Chris never actually said to me: "You're fired." But I began to notice in the listings magazines that he was doing solo shows – just him and a keyboard. They were in the same venues we used to play, then in smaller venues, and then eventually there were no shows at all.

I moved back to London.

Ten years later I was in the park with my son when the phone rang.

"HELLO!" said Frank Sidebottom.

"It's been so long. How are you?" I said.

"Oh I'm very well actually, Mr Ronson," Frank said.

"Frank," I said. "Will you put Chris on?"

Maggie Gyllenhall, Michael Fassbender and Domhnall Gleeson in Frank

Chris filled me in on the past 10 years. Now divorced from Paula, he was an animator on the children's claymation series Pingu. He loved the work but missed Frank and wanted to bring him back from retirement. He was wondering if I'd write something about my time in the band to help him with the comeback. My story was published in the Guardian. My friend, the screenwriter Peter Straughan, asked me if I thought the story could be adapted into a film.

Not long after that, Frank was playing at a pub near my flat. I found Chris in a dressing room at the back, Frank's head in a bin bag at his feet.

"How did you lose so much weight?" I asked.

"I don't know," he said, looking pleased.

"Well, whatever you're doing," I said, "you look great."

We walked across Kentish Town Road so Chris could buy some cigarettes. He'd already given us his approval on the film and I told him the latest news. FilmFour wanted to fund its development. But – and Chris and I shuffled awkwardly around the question – what would the film actually be about? Specifically, Chris wondered, would Chris be in it? Chris had always said we could do what we wanted with the story. But he was worried that however the film might depict Chris, any reality would surely damage Frank.

I had similar concerns. Chris portrayed himself as untroubled. While a total dearth of anxiety was a fantastically enviable character trait in real life, how could we write a film about a man who just didn't care when everything went wrong and in fact found disaster funny? And if Chris was secretly more obsessive about Frank than he let on, how would he feel if the film reflected that? But there was a solution. What if we fictionalised the whole thing? It could be a fable instead of a biopic – a tribute to people like Frank who were just too fantastically strange to make it in the mainstream.

I set off for America to research other great musicians who'd ended up on the margins – Daniel Johnston, Captain Beefheart, the Shaggs. A week after I returned, I saw Frank Sidebottom's name trending on Twitter. I clicked on the link and it said "Frank Sidebottom dead". I wondered why Chris had decided to kill off Frank. So I clicked another link:

Stars lead tributes as Frank Sidebottom comic dies at 54

Chris Sievey, famous as his alter ego Frank Sidebottom, was found collapsed at his home in Hale early yesterday. It is understood that his girlfriend called an ambulance and he was taken to Wythenshawe Hospital, where his death was confirmed.

Manchester Evening News, 22 June 2010

When I'd told Chris at our last meeting how thin he looked – he didn't know it then, but it had been throat cancer.

Frank Sidebottom comic faces pauper's funeral

The comic genius behind Mancunian legend Frank Sidebottom is facing a pauper's funeral after dying virtually penniless. Chris Sievey had no assets and little money in the bank, his family have revealed.

Manchester Evening News, 23 June 2010

A pauper's funeral? What did that involve? A journey back in time 200 years? I sent out a tweet. Within an hour 554 people had donated £6,950.03. By the end of the day it was 1,632 donors raising a total of £21,631.55. The donations never stopped. We had to stop them.

A Timperley village councillor, Neil Taylor, started his own fund-raising campaign for a memorial statue – Frank cast in bronze. He sent me photographs of its journey from the foundry in the Czech Republic to its final resting place outside Johnson's the dry cleaners in Timperley. In the photographs, Frank looked like he'd been kidnapped but was fine with it.

And now our Frank film – directed by Lenny Abrahamson and starring Michael Fassbender, Maggie Gyllenhaal and Domhnall Gleeson, is going to be premiered at the Sundance film festival. As I prepare to go to it, I remember something Chris once said to me. It was late one night, and we were in the van, reminiscing about a show we'd played a few weeks earlier at JB's nightclub in Dudley. It was very poorly attended. There can't have been more than 15 people in the audience. One of them produced a ball, the audience split into teams and, ignoring us, played a game. In the van, Chris smiled wistfully.

"That Dudley gig," he said.

"Ah ha?" I said.

"Best show we ever played," he said.

This is an edited extract from Frank: The True Story that Inspired the Movie. It is published by Picador as an ebook on 16 January. Frank premieres at the Sundance Film Festival on Friday, and opens in cinemas nationwide in May 2014.