Bestselling author Holly Jackson shares her secrets for plotting a modern murder mystery – and explains how true crime has influenced her.

For the author of A Good Girl’s Guide to Murder, the process of writing a whodunnit is as meticulous as investigating a crime.

“I am obsessive about it,” she says. “I don’t quite have a ‘murder board’ because it’s not on the wall, but it is on the floor.”

Each scene in one of Holly’s books corresponds to an index card, which is then carefully placed into columns for each act in the story. The author admits this “does rather take over the room”.

While this is great for planning a storyline, Holly says opening her office door a “bit too ferociously” can literally blow her plot out of place.

A Good Girl’s Guide to Murder follows plucky heroine Pip Fitz-Amobi as she investigates a closed murder case. Pip soon finds a co-detective in Ravi Singh, whose brother was implicated in the crime.

Each clue, twist and turn in the story has been thoroughly discussed by Holly’s fans on TikTok; the hashtag for A Good Girl’s Guide to Murder – #agggtm – has more than 58,000 posts.

And the story has now been turned into a BBC drama by lead writer Poppy Cogan, with Holly serving as executive producer.

The Guardian called the series a “very modern Nancy Drew,” with fans on TikTok praising the show, stitching their reactions with clips from the new series.

The BBC spoke to Holly about the process of writing her hit novel. “Obviously, I love murder,” she says, “fictional murder.”

Holly, 31, from Buckinghamshire, published her debut in 2019. She won a British Book Award the following year and has sold millions of copies around the world.

While her fiction fits into the young adult category, Holly does not shy away from heavier topics, like crime. Her first novel, for example, follows the disappearance and apparent murder of a school girl.

And Holly says true crime content – like the podcast Serial – became a “very useful” tool when writing A Good Girl’s Guide to Murder. The structure of the book feels like a podcast, Holly says, adding: “We have transcripts of dialogue the whole time.”

In the sequel to Holly’s first book – called Good Girl, Bad Blood – Pip even creates a true crime podcast herself.

And Holly says this research tool soon seeped into her real-life. “I can’t really do anything without a true crime podcast,” she says. “If I’m walking the dog or washing the dishes, I need true crime in my ears.”

In the last ten years, true crime series have won international acclaim: Serial won a Peabody Award in 2015 and In The Dark – a long-form investigative journalism series – became the first podcast to win a George Polk Award in 2019. And, according to The New York Times, Serial has had more 705m downloads.

Even Holly is curious why crime is such a popular source of entertainment.

“Especially with young women,” she wonders, “is that like, an instinct in us that’s trying to protect ourselves?”

Georgia Hardstark is the co-host of My Favorite Murder, a US podcast that looks into historic and modern cases, with one episode covering the Dancing Plague of 1518 and the Paper Bag Killer.

For Georgia, part of the reason she is so interested in true crime is that it helps her feel less “paranoid” and validates her anxieties about life, she explains.

“That is at the forefront of my mind, constantly, you know, ‘What’s around the next corner? Are my doors locked?'”

For Holly, the line between fact and fiction is clearly drawn: unlike true crime cases, she always knows “the ending before I even write the first sentence”.

“I knew from the get-go who the murderer was going to be, this whole setup,” she says. “The slightly more complicated thing is not working out the mystery – it’s working out how Pip is going to solve the mystery.”

In A Good Girl’s Guide to Murder, for example, Pip uses her Extended Project Qualification – an accreditation where a student independently researches a given topic – to interview suspects and keep track of clues for the case.

While Holly uses true crime as a “jumping off” point for research, she notes the content, often used as a source of entertainment, is “obviously, about real life people’s trauma”.

Jessica Jarlvi – a “Scandi-noir” writer and lecturer on the University of Cambridge’s Crime and Thriller Writing course – says things like true crime podcasts risk sensationalising these events.

“It just puts me off,” she says, “whereas in fiction, you don’t have to worry about that.”

In Georgia’s view, however, ignoring real-life crime – often with women victims – “is to sweep it under the rug”.

Modern crime readers are “becoming more and more demanding”, Jessica adds.

Holly agrees: “I don’t have those passive readers, I have the really active ones who are looking to solve the mystery.”

On TikTok, fans of A Good Girl’s Guide to Murder share videos with their predictions and suspect lists as they read along with the book.

In one video, a reader guides people on how to annotate the book to keep track, colour co-ordinating sections into “clues” and “conflicts”.

“It makes me have to up my game a bit more,” Holly says.

Wondering how to watch A Good Girl’s Guide to Murder? You can stream the series on BBC iPlayer.