'Who better to be a detective than someone who is invisible, but incredibly wise?'

Richard Osman’s hugely popular series of books are set in an English retirement village where four residents investigate unsolved murders.

The first of the series has been adapted into a Netflix movie, Thursday Murder Club, starring Helen Mirren, Pierce Brosnan and Ben Kingsley.

The Impossible Fortune is the fifth book in the series, and 54-year-old Osman says the company of his four amateur detectives - who've been described as "senior citizen X-Men" - continues to inspire him.

"I love spending time with the characters, so long as readers love spending time with the characters, then the job is done," he told RNZ’s Saturday Morning.

Osman, a successful television presenter and producer, believes the success of Thursday Murder Club lies in our fascination with secrets.

“We all love secrets; we all love what's behind a closed door.”

That and the satisfaction of seeing problems solved in a world seeming full of intractable problems adds to the attraction, he says.

"The great thing about a crime novel is we all know when we're reading them, the author is going to show us something impossible at the beginning, and is just going to say, oh my God, there's no way we're going to get out of this mess. But you know, at the end, we will get out of this mess. That's a very comforting thing."

When he sat down and started on the first novel, which came out in 2020, he wanted to create a gang, albeit one that had plenty of years under their collective belt, he says.

"My favourite books when I was a kid were the Famous Five books. My favourite TV show when I was a kid was The A-Team. And I like the idea of having unlikely people and putting them together, because there's a huge amount of humour in that."

It's been a winning formula with global sales for the series topping 10 million.

And his gang are four people from a part of society that seldom enjoys the spotlight, he says.

"I realised very early on that I had a group of people who’d really lived a life and so had interesting things to say, and had lived different lives, so would have interesting things to say to each other, but had this invisibility that was underestimated.

"And you think, well, who better to be a detective than someone who is invisible, but incredibly wise?”

He’s been hands-off with the movie based on the first novel, he says.

"I just get the fun bits, I get the premiere, I get to visit set, it's like my grandchild, rather than my child, I get the fun stuff, but I get to hand it back at the end of the day."

When Steven Spielberg acquired the film rights early in the piece, it gave him the belief he was on the right track, he says.

"He bought the rights to the book before the first one even came out. Which gave me so much confidence, because, forget where it's from, the thing, he understands is story and character and warmth.

"And when he said that he loved the books, that's the first time I really thought, oh my God, there might be something in this Thursday Murder Club."