'Don't be daft," Steve Cavanagh says when I ask the Belfast-born author if he would ever kill off Eddie Flynn, his conman-turned-New York-trial-lawyer.
Flynn has featured in five internationally successful novels, so his creator's reluctance to dispense with him is understandable. But for many crime writers, returning to the same lead character can be a challenge, and an unexpected one.
"I don't think anyone can honestly say they envisaged a long-running series, because it's not really up to the writer," says Cavanagh. "I did have an idea for a second book [when writing the first], but I was no fool. If the first one wasn't going to get published, there was no point in writing a second."
Sinéad Crowley's debut novel Can Anybody Help Me? became a trilogy. "My first book was about a young, lonely first-time mother who got involved with mysterious goings-on on a parenting website," says the RTÉ correspondent. "In the first draft, that woman solved the mystery herself, but that wasn't sustainable, so I needed a garda. As soon as I started to write about DS Claire Boyle, she emerged on the page as a fully formed, sweary, determined character and I loved her immediately."
She and Claire are on a break while Crowley writes a standalone novel, but she wants to return to her. "I had a definite story arc in mind and I want to see it come to fruition," she says.
Andrea Carter's fifth Inishowen mystery, The Body Falls, was published in April, but she did not start off with a series in mind when she created solicitor Benedicta 'Ben' O'Keeffe. Her first book was written purely for herself while living in the Donegal peninsula and running the most northerly solicitors' practice in the country, offering the "last legal advice before Iceland". "I created a character who was doing the same, but who could say things I couldn't and behave in a way that I wouldn't. It was a kind of therapy," she says.
The Inishowen series is being adapted for television, bringing a new dimension to Carter's feelings about Ben. "It's a strange and exciting prospect and I really don't know how I am going to feel… I imagine she will change for me when I see her on screen and become someone I don't know quite so well," she says.
When a book evolves into a long-running series, the speed at which the protagonist ages can become complicated. On Hercule Poirot's first appearance - in The Mysterious Affair at Styles, published in 1921 - we are told that he retired from the Belgian police aged 55 in 1905, yet Agatha Christie didn't bring the curtain down until 1975. Ian Rankin's Inspector Rebus had his first outing aged 40 in 1987, and was most recently spotted 22 books later, giving the author the thorny problem of a post-retirement age policeman who can no longer get into fights because he would lose.
Ageing is an issue, but so is personality. Christie grew to despise the quirks she had loaded on to Poirot. She introduced a second character, Ariadne Oliver - a crime writer with a long-running series featuring a vegetarian Finnish detective - to express her frustration. She created a man who was fussy, vain and prim, only to find herself stuck with a "detestable, bombastic, tiresome, egocentric little creep".
Jane Casey was aware of this when she created Maeve Kerrigan, a London-Irish Metropolitan Police detective. "Initially, I wanted her to be quite normal and ordinary, so she doesn't have any annoying hobbies or traits that I have to include in every book; because Maeve was early in her career in the first book, she's had room to grow personally and professionally," she says.
"She has definitely changed - the things she's seen and done have affected her - and that's intriguing to write about."
Casey is planning her 10th Kerrigan novel. "Things could change for Maeve at any moment and none of my plans for her are set in stone… I hope and intend to write Maeve novels for a long time to come," she says.
But what happens when an author does want out? "I must save my mind for better things, even if it means I must bury my pocketbook with him," Arthur Conan Doyle wrote to his mother, believing Sherlock Holmes had become a tiresome distraction. The 1893 Strand Magazine short story 'The Final Problem' was supposed to spell Holmes' demise, apparently having plunged to his death at the Reichenbach Falls with his arch-enemy Moriarty. Public pressure meant he escaped Doyle's attempt to kill him off, however, and the detective's final appearance in print came 24 years later.
Fans become invested in a long-running series and to keep the relationship interesting, authors must find increasingly inventive scenarios without breaking the bonds between reader and character. While Cavanagh continues to develop Eddie Flynn, he is careful to stay within the confines of what he originally established. "If I suddenly departed from that, I don't think it would work - it would just be bad writing."
Casey agrees: "Readers have very strong opinions about Maeve and how her life should develop, which is enormously reassuring as I know they see her as a real person. As long as you stay true to the characters, you can bring the readers with you, whatever you do with them."
Repeat characters are often police or private detectives because such professions put them in the line of trouble, but with a 'civilian' character such as solicitor Ben, the challenge becomes harder. "I've started importing people to kill," Carter says. "Inishowen is a place which draws visitors, so it works well. Outsiders come to live there, people with secrets and mysterious pasts they wish to conceal."
Luckily for crime fans, secrets always leak out.
Henrietta McKervey is author of 'A Talented Man', published by Hachette