Mr Selfridge actress Katherine Kelly lives every mother’s worst nightmare in ITV’s new three-part drama The Guilty (Thursday, September 5), playing a woman whose son’s body is found five years after he went missing.

Here Katherine explains more about the drama – and how she cried for weeks!

What can you tell us about Claire Reid, the character you play in The Guilty?

“Claire’s definitely strong to still be upright and functioning after all she’s been through. I hope she’s someone that everybody can identify with and feel for because it could happen to you or someone you know.”

Do Claire and husband Daniel (played by Spy star Darren Boyd) have a strong relationship?

“The fact they’re still together after five years of pure purgatory shows they are a pretty strong couple, because it would have broken many couples. But the marriage definitely has cracks – it’s not this chocolate box, Pleasantville relationship; you see things are not quite right.”

Leading the hunt for their son Callum’s killer is DCI Maggie Brand (Tamsin Greig), who was forced to step down from the original investigation into the boy’s disappearance because of severe morning sickness. How does Claire respond to Maggie?

“It’s not like they have a bond but they do have this connection. There’s a sympathy as mothers, but it doesn’t go any further than that because Maggie’s the professional and Claire doesn’t want a new best friend – she just wants to find out who killed her son.”

Did you do any research for the role?

“I know people do, but I tend not to as it doesn’t really sit well with me – I feel like I’m taking from someone’s misfortune for my own personal gain. I’m a big reader and no matter what I’m doing I like to read around it and connect the dots.”

Were you able to really put yourself in the shoes of a mother who’s lost a child?

“It’s such an extreme circumstance, so how can you possibly say how you would act in that same situation? There’s no right or wrong answer – you’ve just got to tell the story you’re trying to tell and make the best decisions for that story.”

As the subject matter is so heart-wrenching, was it hard to switch off after a day’s filming?

“I’m not like that – I’ve never been a person that takes things home. As soon as they shout ‘cut’, I’m like: ‘Can I have a cup of tea?’ But, physically, it takes a toll on your body. By about week three I’m thinking: ‘Why are my eyes still swollen?’ It was because my body had been crying for three weeks!”

What sets The Guilty apart from other crime dramas?

“If you had to label it, you would say it is a crime drama, but it’s so much more than that – it’s not all wrapped up in a box with a bow and label stuck on it. To me, it’s really about two women trying to find their sons in very different ways.”