'Everyone thinks we're brother and sister in real life' – Tom Riley and Charlotte Riley talk playing siblings in ITV's Dark Heart

If they share a surname, Charlotte says, why not play siblings?

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"In actual life everybody, casting directors and people in the industry, think that Tom and I are brother and sister," she tells us.

"It's quite unusual, 'Riley' spelt like that. So that's one of the reasons I did the job! I only did the job so that I could actually play his sister for real."

To make matters more confusing, Charlotte Riley is actually married to a Tom. In 2014 she married the actor Tom Hardy.

(Tom Riley is not married to a Charlotte; his wife is the actress Lizzy Caplan.)

Now that they are working together, the two Rileys come up with a backstory worthy of its own TV show.

"Charlotte and I, we joke that we were part of an experiment and that we were separated at birth," Tom says. "One of us was raised in the north and one of us was raised in the south, and we both ended up in the same show – but didn't know any different."

That show, Dark Heart, stars Tom as a detective haunted by the memory of his parents' brutal murder. Losing his mother and father at the age of 16 has left him unable to form solid relationships, but it has driven his police career and his "speciality" interest in particularly ugly crimes.

His one enduring relationship is with his older sister Juliette, who also bears the emotional scars of losing her parents so young. But however much the siblings love each other, they also frequently clash, particularly as Juliette clings to her abusive fiancé Paolo.

"I think they only have each other," Tom says. "They are each other's only living adult relatives and they are the only ones who can understand what the other one's gone through, but they have very different lives and they rub each other up the wrong way all the time."

"They've both been hugely affected by the loss of their parents which is kind of the undercurrent for what's happening for Will and Juliette," Charlotte adds. "And those two people have dealt with that in very different ways. She channels it all into her family and her love life, and he's channeled it all into work."

When it comes to Wagstaffe's relationships, Tom explains, "any form of intimacy is tricky for him. And that's why he can't hold down this relationship properly, why he's not good with his partner, why he's difficult with his sister.

"But the only pure form of intimacy that he truly understands is his nephew, which is Charlotte's son, and he's able to get intimate there because it's unconditional. The kid's not going anywhere, has to be there, so he's able to open his heart to him.

"Everyone else, the minute there's some kind of emotion – he's looking for the doorway."

Dark Heart will air on Wednesdays at Thursdays at 9pm on ITV

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