BEST-SELLING Scandi-noir author Jo Nesbo was never one to be pigeonholed.
Now on a whistle-stop tour of Europe to promote his latest thriller, Knife, he's only just returned from a series of gigs at Norwegian festivals where he was performing with his rock band Di Derre ('Those Guys') who were massive in Norway in the 1990s.
The 59-year-old rocker, creator of the hugely popular tortured Oslo detective Harry Hole novels, has also penned a series of lighthearted children's books, although it's his crime-writing which has brought him literary fame.
To date, the Norwegian author's spine-tingling thrillers – including The Leopard, The Snowman and The Thirst, featuring his serial killer-chasing alcoholic detective – have sold more than 40 million copies worldwide and spawned a couple of movies.
They take readers on tortuous journeys with depraved killers and their terrified victims, down Oslo's dark backstreets and beyond.
Today, Nesbo is far more upbeat than his dour detective hero – although he admits he is already planning the end for Harry, but won't reveal in which book or how he will be written out.
"I think there's a future for Harry. At one point, I wrote a storyline for his life and we are getting close to the end of that storyline," he reveals.
"But I can't tell you how many novels it will take to get him to the end of that future. I can't tell you if I'm going to kill him off – that would be a spoiler. Let's put it this way, after the end, he will not resurrect."
After 12 Harry Hole novels, Nesbo admits he has made himself ease up on his notoriously graphic torture scenes since The Leopard, arguably his most violent novel, in which a victim had a spring-loaded ball stuffed in her mouth which shot out spikes.
"In that book, I went too far," he admits.
"The novel had good reviews, but one Swedish reviewer was furious about the level of violence and I had to reconsider. When I re-read the pages he referred to, I realised he was right.
"There is still violence, but it's much less graphic than it was in The Leopard. I will use violence only to the extent that it is needed to describe the characters and their emotions, and to show the reader what's at stake."
He may have pared down the detail but he's still racking up the attacks and killings in Knife, in which a murder with deep personal ramifications sees Hole once again pursuing serial rapist and murderer Svein 'The Fiance' Finne, who is now out of prison and seeking revenge.
The killer has an impressive collection of knives and it was while reading a book by an American psychologist about what stops us killing each other which drew Nesbo to the theme. He discovered that, while you could train soldiers to shoot at the enemy, it was much harder mentally to stab someone to death,
Nesbo explains.
"The one area it was very difficult to get soldiers to kill was close up. As long as you don't see your enemy's face or see them as human beings, it's quite easy to manipulate the brain, but when you get close with knives it's very difficult.
"Why is it that the closer you get, the more difficult it is to kill someone? That was the starting point for the novel.
He continues: "It's harder to kill someone with a knife, but it's also probably the weapon we are most afraid of. There's more horror in being penetrated by a piece of steel in the form of a knife than with a lead bullet."
Although two of his books, Headhunters and The Snowman, have been adapted for screen – Headhunters received rave reviews, while The Snowman (which starred Michael Fassbender as Harry Hole) was panned – Nesbo is in no hurry to get involved in film-making. He hasn't even seen The Snowman.
"I was in Greece rock-climbing with friends when The Snowman premiered in Oslo. I read the reviews, which were not good, and I didn't get around to seeing it. So now, when people ask me what I thought of the movie, I don't have to criticise anyone's work."
Away from his writing, Nesbo lives in Oslo, and aside from saying he enjoys climbing, music and watching movies. He refuses to discuss his private life and only mentions his daughter, Selma, occasionally.
Once set for football stardom (until he tore the cruciate ligaments in his knees at 19), he then had a 10-year career as a stockbroker followed by huge pop success in Norway with Di Derre.
Nesbo went into writing relatively late, at 37, when a publisher asked him to write a book about a pop star's life on the road. Instead, Harry Hole popped into his head on a flight from Oslo to Sydney and his first crime thriller was born.
Does he become as depressed as his detective hero when he's writing about him?
"I don't feel like my fiction spills over into my real life but I spend so much time in Harry's universe that at the end of writing a novel like this, I need a break from that world and from Harry himself," he admits.
"Harry's the kind of friend that if you spend a weekend with him you appreciate it, but you don't call him back on Monday."
Certainly, the Harry Hole stories are a world away from Nesbo's children's books, The Doctor Proctor's Fart Powder series, about the adventures of an eccentric scientist who dreams of becoming a famous inventor, aimed at seven-year-olds.
"They are just different stories," he says, with a shrug.
"Basically I'm a storyteller. Before I wrote crime stories I wrote lyrics for my band – that was my school of writing. They stimulate my creativity."
When he returns to Norway, he'll be rejoining his band to perform three gigs with the Oslo Symphony Orchestra near the ski-jumping area of the Holmenkollen.
He laughs at the notion that he still considers himself a 'pop star'.
"We like to think we headline at gigs, but sometimes other bands also think they're headlining. We really just do it for fun."
:: Knife by Jo Nesbo is published by Harvill Secker, priced £20
