The Mrs Biggs star gave birth to her first child, Billy, in 2020 and says even though he is still small, the way she reacts to playing a mother has completely changed because of him.
She stars as Kathy in ITV drama No Return, where she goes on a family holiday to Turkey but is devastated when her teenage son Noah (Louis Ashbourne Serkis) is jailed for a crime he says he didn't commit and she must battle with an alien legal system to try to free him.
Watch: Sheridan Smith takes on harrowing role in ITV's No Return
Calling it "the right role to play after having a child", Smith, 40, said: "I’ve played a lot of mums but this is the first time I’ve played a mum as an actual mother myself. It’s so different.
"You can act being a mum but there’s something very different when you are a parent yourself with a child of your own.
"All I kept thinking of was Billy. Every time that would just set me off. I didn’t have to act too much in the crying scenes because you just imagine this mother going through this hell and thinking if it was my little boy. So I could totally relate to this.
"It felt like the right role to play after having a child and being a mum and really understanding the lioness of a mother that I would be and every mum would be, I’m sure, and every parent would be to try and save their child."
Smith said the drama would be tough to watch for many, and added: "It was harrowing to film. I’ve done lots of different roles but this is just like a hell ride. You don’t really come up for air until the end."
The actor has starred in acclaimed dramas including The C Word, Cilla and Cleaning Up, but said having Billy had changed the way she worked.
She said: "Becoming a mum was a great thing for me. I’m not selfish anymore. I don’t go home constantly thinking, 'Was that scene all right?' I just want to go home and see my son.
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"It’s also those feelings you have as a parent. Those primal, protective feelings. It’s a love like no other. People had told me that and said, 'You won’t know until you have your own.' They were so right. It’s like a switch being flicked. Everything changes for the better.
"But you can never go back. You’ll never be carefree. You just worry about this little one so much."
Smith also said she hoped to be able to take Billy with her every time she was away for work as he had travelled to Spain with her where No Return was filmed, although it is set in Turkey, but had been unable to take him on another job to Budapest for six weeks because of coronavirus restrictions.
She said: "I missed him so much. So I hope if I do jobs in the future I can always have him with me. Even if it’s just for a little bit of it."
The star added that employing a nanny had enabled her to work: "I think people sometimes are embarrassed to say if they’ve got a nanny. But if I’m working full time there’s no way I could have him on set with me with him running around like he does at home.
"She was brilliant. I was very grateful to her because I wouldn’t be able to do the job otherwise."
No Return begins on Monday, 7 February at 9pm on ITV.
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