In adapting any book to TV, there are some difficult decisions to make. Which characters will you keep, and which will you cut? How much of the plot can you include in a more limited time period? And what just won’t work in a completely different medium?

And when adapting JK Rowling’s Cormoran Strike crime novels (written under the pseudonym Robert Galbraith), the team behind new BBC series Strike also had to contend with the pressure of bringing another property by the Harry Potter author to screen. Not the easiest situation to find yourself in….

“I think there are a number of challenges with adaptation,” Ben Richards, who wrote the first three-part mystery for the series based on Rowling’s The Cuckoo’s Calling, said at a recent preview screening.

“It’s to keep the essence of the book, to keep the essence of the story, while making it suitable for the slightly different demands of television in terms of pace and shape and stuff like that.”

And so a few alterations were made to Rowling’s original story. While nothing major is changed in the Cuckoo’s Calling (for example, it’s still the same overall plot and the same murderer, and Strike’s backstory is the same), minor details such as the name of victim Lula Landry’s driver (changed from Kieran to Nico), the background of her boyfriend Evan (switched from being an actor to being a rockstar), the rank of policeman Eric Wardle (promoted from DS in the novels to DI) and general ordering of events from the novel are all modified.

Elsewhere, more extreme changes can be seen in the deletion of entire characters from the narrative, a choice explained by Richards as a consequence of TV’s limitations.

“By necessity you have to have fewer characters, just to make it easily accessible, particularly in a first episode,” he said. “If you throw in too many characters you can end up with quite a lot of confusion.

Axed characters from the three-part Cuckoo’s Calling adaptation include Alison Cresswell (the girlfriend of Lula’s brother John), lawyer Cyprian May and his wife Ursula, while others (like aforementioned driver Kieran/Nico) have their roles considerably scaled back, or even modified – the DI Wardle of the TV adaptation is a slightly less helpful figure than the book’s version, for example.

There are also certain scenes (like a birthday party for Strike’s sister Lucy and a meeting with some of Strike’s old friends) that don’t make the cut, though it’s possible some of those settings and characters will appear in subsequent sequel adaptations (the next Strike novel, The Silkworm, is adapted in two parts later this month).

Still, perhaps the biggest change from the books is a largely visual one. When casting Tom Burke as Strike, a character described as considerably taller and bulkier than the Musketeers star, the team behind the adaptation knew they’d have to work around it.

“Tom acted big,” director Michael Keillor said. “There were tricks – we wanted to give him some heft. But I think, once you’re watching something, once you get into it you’re not thinking ‘is he 6 foot four or 6 foot two?’ You just go with it.”

“[The book] talked about his hair being wiry like pubes,” he went on. “That wasn’t something that we thought would communicate as well on screen…”

“I did offer to get a perm!” Burke added.

“We’re also trying to create something that’s a bit different to the book, something that’s new,” Keillor concluded.

“And it won’t be compared directly – it’s kind of a companion piece.”