Cormoran Strikes again! The private eye finds his assistant Robin menaced by one of three foes from his past in the latest gripping JK Rowling adaptation

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The unlikely partnership between down-at-heel private investigator Cormoran Strike and his feisty female sidekick was a huge hit for the BBC last year – and the latest adaptation of JK Rowling's bestselling books will see it hot up even more.

Dark, fast-paced and compelling, the BBC's first two adaptations of the Harry Potter author's detective novels struck a chord with viewers late last summer. 

Nearly eight million tuned in to The Cuckoo's Calling and The Silkworm, which introduced us to Strike, a former Army special investigator with a troubled past who lost his right leg below the knee in Afghanistan, and his colleague Robin Ellacott.

Operating out of a dingy Soho office that doubles as his digs, Strike's a dour character with a sharp mind who likes a drink, while Robin's a prim but practical former psychology student engaged to her university sweetheart. 

Private investigator Cormoran Strike (pictured with his assistant Robin) returns in a third BBC adaptation to investigate a serial killer

She started as Strike's temp for a week but her enthusiasm and intelligence impressed him enough to keep her on as his assistant. And from the start it was clear the flirtation between the pair – played by Tom Burke and Holliday Grainger – was as key as the gritty plots set in London's seamy underbelly.

'We wanted there to be the feeling that there was something between them without them pushing it,' says Tom, who made his name as Athos in The Musketeers and womanising cavalryman Fedya Dolokhov in the BBC's War And Peace. 'So it's present, but half the time they're trying to pretend it's not.'

The third adaptation, Career Of Evil, kicks off in gruesome fashion when a suitcase containing a severed leg is delivered to a horrified Robin, along with a note quoting lyrics from a song by Blue Oyster Cult, a US rock band that was a favourite of Strike's mother. 

Strike's father was a rock star who neglected him and his mother was a super-groupie who died from what was thought to be an accidental overdose. Strike remains convinced she was murdered.

This very personal taunt leads him to believe he knows the killer and he's out for revenge, but the problem for Strike is that it could be any one of three men, including his stepfather Jeff Whittaker (played by Matt King). 

JK Rowling wrote the novels featuring Strike under the pseudonym Robert Galbraith and last year it was turned into a hit series

The murderer then discovers where Robin lives and begins to stalk her, and Strike becomes concerned for her safety. So too does her fiancé Matthew, but when it's revealed he slept with another woman at university they cancel the wedding.

When another body part turns up, it's clear they're dealing with a serial killer. Strike and Robin drive north on the trail of Donald Laing, the detective's nemesis since their Army days.

'There's a moment when Cormoran and Robin are staying in a motel,' says Tom. 'It has that frisson we haven't had before where anything could happen. But you still feel they're in denial of it a bit.'

You can certainly see Robin's appeal for Strike. Hot-headed, brave and beautiful, she's well able to hold her own. 'There are moments where Strike tries to dissuade her from plunging in and moments where she just does it off her own bat,' says Tom. 'But he increasingly feels confidence and pride in her as he gets to know her better. He's very protective of her and she is of him.

'Holly and I worked even more closely together on this story. There are moments that need real tenderness and pain, and it's nice to feel you're acting with someone. It's about trust, which is why it went so smoothly.' Holliday, 29, agrees. 'The nature of the crime brings out more of their back stories that not only do the viewers not yet know, but they don't know about each other either. So it forces them to be a lot more open with each other. And with Robin and Matthew kind of splitting up, it allows her to have a closeness with Strike.'

So does she think Robin and Strike will end up together? 'I don't think she's quite aware of her own feelings for Strike,' says Holliday. 

'I think her feelings for him are caught up in her feelings for the job, and a sense that she's discovering herself through the work. Strike's the first person who recognises that in her and has respect for her. You'd fall for that, whether or not there's anything romantic behind it.'

JK Rowling wrote the novels under the pseudonym Robert Galbraith but was revealed as the real author in 2013. 'I had this dream that I might get three books out before anyone realised it was me,' she explains. 'But things became difficult quickly because the book was picked up by a publisher. Then the BBC wanted to adapt it before they knew it was me. That presented 'Robert Galbraith' with his biggest problem,' she laughs.

Rowling was a fan of Tom Burke having seen his portrayal of Dolokhov in War And Peace, her favourite character in Tolstoy's epic novel. A further connection is that Tom is from an acting family, and his godfather, the late Alan Rickman, played Severus Snape in the Harry Potter films.

Tom Burke who plays Strike (pictured with his stepfather Jeff) says being offered the role felt like winning the lottery

When Tom landed the role of Strike he said it was like winning the lottery. 'When I read the first book I was so impressed,' he says. 'The detail and the research she goes into as a writer.' 

He's impressed with her hands-off approach to the TV versions too. 'She's very involved up to a point and then we just get on with it,' he says. 'I think she enjoys watching and being surprised in some places, knowing we've done what she felt was important.'

To handle the tough running and walking sequences with a prosthetic leg, Tom had advisors on set and took notes from soldiers who had fought in Afghanistan. One scene involved chasing someone through the streets of Soho. 'The leg isn't built for running. You'd only run on it if it was life or death, which in the show it is.'

He enjoys the fight scenes too and bulked himself up for the role. 'Sometimes you do wake up in the morning thinking, 'I've got a whole day of fighting,' because they film the scenes from every angle. But it's a nice challenge. These scenes are rarer too, because Strike does a lot of talking!' 

Strike – Career Of Evil, tomorrow, 9pm, and Sunday 4 March, 9pm, BBC1. Strike – The Silkworm, the second adaptation, is available on DVD now.