In new thriller The Sister, the brilliant Russell Tovey plays a man with big secrets. He talks to Georgia Humphreys about his experience on set.

Russell Tovey doesn't gloss over how challenging his latest TV role was.

In fact, the Essex-born actor declares The Sister "is the hardest job I've ever done".

He plays Nathan, who is completely riddled with anxiety and fear and unable to escape his past, in the four-part ITV series.

"It's about the unravelling of a good person who made a mistake, or got involved in a situation he shouldn't have been involved in," Tovey, 38, elaborates of the storyline.

"And to commit to that as an actor, when every single scene is a panic attack or anxiety or screaming or crying, and you're in every single scene, it's really tough.

"I feel like, as actors instinctively, you trick your body into feeling these emotions. You're tricking your body into fight or flight and you're doing it over and over and over again."

The Sister was written by Luther creator Neil Cross, inspired by his novel Burial.

Nathan has never been able to forget the worst night of his life: a party that led to the sudden, shocking death of a young woman.

Only he and Bob, an eccentric old acquaintance, know what really happened... and they've resolved to keep it that way.

But, years later, Bob - played by Doctor Foster star Bertie Carvel - appears at Nathan's door with terrifying news that threatens to tear Nathan's world apart.

"Nathan's got everything compartmentalised by this point," notes Tovey. "He's able to live. He wants to live and he's full of life and he loves his wife and all he wants is to start a family and be normal. It's all he ever wanted.

"He doesn't have big dreams. He just wants to be happy. And Bob turns up and he unravels."

Tovey - who's also starred in dramas such as BBC's Being Human and Years And Years, and ITV's Flesh And Blood - reflects on how different The Sister was from previous projects.

"I am someone that, on set, I like to have a laugh, I like a bit of banter, talk to the crew. This one, I was just in a little coffin, in my head, trying to hold it all in. Because it's a genre piece, it's beautifully shot by Ben Wheeler, the DOP(director of photography) is a genius.

"It's visually stunning. But to set that up is a long time and then you've set it up and then you're straight in doing these scenes where you're just falling apart."

So, what did he do to help himself through the intense shooting process?

"You've just got to look after yourself. I had loads of vitamins, like fads. I was doing Berocca with CBD shots every morning and then I found this new thing where you put fresh lemon and orange in this bit in the middle and you put water in it and drink that all day. Just things to make sure that you're being good to yourself."

He adds: "All you can do on any job, for me personally, is you've just to give it your all. Because if you walk away from that being like, 'I could have done that better' then that's something that you'll never be able to change."

Tovey has been open in interviews in the past about how he's a "worrier", and how he has various anxieties about himself and being a performer.

Asked if there was anything from his past that he could draw on for Nathan and his struggles, he muses: "You make mistakes in your past. I mean, I've not been involved in the death of a young lady. But as humans, we're all flawed. As humans, we've all said something that's upset someone.

"When you're at a party you say something and you go home and you're like, 'Oh God, I think I really upset that person, because I said that.' And then you live with it for a while and you feel really guilty. And then you see them again and you apologise and they're like, 'I don't remember.' And you're like, OK, it's all in your head.

"You fixate on things. So I guess I'm drawing on a feeling - but a lot of it's on the page."

The star describes Nathan as "a really decent, kind, normal man. He's the everyman."

When it comes to how viewers will react to him, he says: "I think people are really gonna... their hearts are gonna break for him. That's what I want. I want their hearts to break for him and I want them to be conflicted. That they're gonna watch him and be like, 'I can't believe what I'm witnessing that you've done. I really care about you.'

"Every time I do a job, I want my mum to like the character. And I want my mum to want to give them a cuddle. And I think with this character she's gonna be like, 'I really don't like what's happened, but I'm gonna give you a cuddle anyway.' I think it's got that sort of energy."

On the topic of his family, he says his parents read the book The Sister is based on.

"My parents read everything," follows Tovey. "When I did The Good Liar [2019 film with Helen Mirren and Ian McKellen] they read The Good Liar.

"I normally like to have had the script and then I go by the script. And then I have to build my own emotional back story.

"Will I read the book after this? Maybe. I feel like, with this character, when I finished this job, I had to push Nathan away for a while, because it really got under my skin."

The Sister starts on STV on Monday, 9pm.