Daniel Kraus first watched George A Romero’s seminal 1968 horror film Night of the Living Dead when he was five years old. Not the ideal viewing age for a zombie classic satirising consumerism, perhaps, but his mother was loving it. “She was having such a good time, she wasn’t hiding behind the sofa. She was laughing and enjoying it, so I wasn’t too scared,” he says.

A huge fan ever since, it was “stunning, and a little bewildering” when, a month after Romero’s death in 2017, Kraus got a call from the film-maker’s manager, Chris Roe, and Romero’s wife, Suzanne Desrocher-Romero. They wanted Kraus, by now a bestselling novelist whose work includes co-authoring adaptations of The Shape of Water and Trollhunters with Guillermo del Toro, to complete The Living Dead, an epic zombie novel Romero had been writing, on and off, for more than a decade.

“I intend to keep [the chapters] coming until I’m diagnosed as terminal, at which point I’ll, real quick, whip up an ending,” Romero once wrote on his website. In actual fact, when he really was diagnosed as terminal, all he wanted to do was to be with his family; Romero died three months after his diagnosis with lung cancer.

“I wouldn’t even call it a dream come true because I had never even dreamed of it. It was beyond anything I would have thought possible,” says Kraus, of the call. He had once met Romero, 10 years before he died, having been introduced by Roe, who went to the same high school as Kraus and remembered him as a fan. “We just chit-chatted and I didn’t muck up the works by telling him how much his films had meant to me. I played it cool. But I did get a photo out of it,” says Kraus; that photo is in his office today.

Kraus took the job, but it wasn’t straightforward. Romero had written about a third of the book, but not the first third. Kraus was given “plenty of pages” Romero had written, then uncovered 100 more pages that the film-maker once uploaded online – then, later still, found some of his notes detailing where the novel was heading.

It was, Kraus admits, a daunting process. “It would have been a lot easier if he’d written the first half of the book and stopped – someone passes you the baton and you run with it. This was much more complicated, because I treasured what George had written and I didn’t want to lose anything. But there were all these gaps, and some of them were huge,” he says.

Kraus decided to go back to the beginning. He rewatched all of Romero’s zombie films and read the original screenplays. He met with Desrocher-Romero to discuss how to carry on in Romero’s spirit. What began to emerge, he says, “from what George had left, was that this novel was going to be an endpoint”.

Romero had started his zombie story with Night of the Living Dead, then told it over six films over five decades, from which “a lot of his ideas got cut for budgetary reasons and other creative reasons”, says Kraus. He believes that the manuscript of The Living Dead was where Romero would go to “pour all of his ideas that just weren’t gonna fly on screen because they would never give him a big enough budget. It became his kind of solace, where he put, I think, a lot of his passionate thoughts. I think it was intended as two things: one, the zombie epic that he would never have the budget for, and two, a way to end a conversation that he had started 50 years before.”

The Living Dead starts on day one of a zombie crisis as bodies begin waking up in refrigerators, and humanity attempts to hole up and hide away. Getting into the gore was a surreal experience, he admits – it was not something he had ever aspired to do, despite his love for Romero’s work. “I don’t generally write anything that would resemble the kind of gory bacchanalia that you might have with a zombie battle. So I got to exercise some muscles that I don’t normally get to use.

“It was so much fun. I was like a kid in a candy store. I could take take up where Romero had left off, which is writing stories that weren’t about the zombies at all,” he says. “Romero was never particularly interested in the zombies. He made that very clear. He was interested in the failings of humanity. The zombies were just the catalysts. He accidentally created the zombies with Night of the Living Dead, and he ended up getting stuck with them. Once he realised that, he thought he might as well use them to tell stories he would have told in other genres.”

Kraus is careful always to describe Romero’s zombies as being part of an uprising, not an invasion. “It’s the story of a underclass, right? Out to topple a rotten ruling class,” he says. “Romero’s sympathies are with the zombies, not the people! He was rooting for the zombies. It’s one of many things that I love about his stories. He was a 60s radical who never let go of that, a fighter for justice and equality. He really hated anything that drove us apart. And every single one of his stories is about divisions in society. How, even when you have a group of good, intelligent people, after a while a couple of voices will rise up and the group becomes divided.”

Kraus believes Romero would not have been surprised by what’s happening in the Covid-19 pandemic, particularly in the US. “He would have said, ‘Of course we’re not going to be able to work together on this.’ He believed that the zombie threat should have been taken care of in a couple of weeks, we should have been able to handle them. In the same way, we should have been able to handle a global pandemic, but we couldn’t.”

Now Kraus has finished with it, The Living Dead is a meaty zombie epic spanning the US. It includes characters such as Charlene, known as Charlie, who works in a morgue and, with her boss Luis, is the first to witness the terrifying awakening of the undead. There’s Greer, a teenager in a midwestern trailer park. There’s Etta, an autistic federal employee who is charting the outbreak in DC, recording “what was happening so that future denizens might find evidence of the grand, complicated, flawed-but-sometimes-beautiful world that existed before the fall”.

Romero’s widow, Suz (“the only review I needed”) loves the finished novel. Kraus, meanwhile, has made his peace with what Romero’s army of fans will think.

“I have realised that if the book is good, people are going to say hurrah for George. And if it’s bad, they’re gonna blame me,” he says. “It may not be what a casual fan is looking for, because it is not 700 pages of zombie action, because George would have hated that. The deeper fan, who understood what his concerns were, I think will get a lot out of it, and so will people who don’t know anything about George. The only audience I’m worried about is that sliver of fandom that’s just in it for the gore. It’s there, but you’re gonna have to find it.”

• The Living Dead by George A Romero and Daniel Kraus is published by Bantam Press.