The actor says she feels protective mum roles much more keenly now that she has her own child.
The upcoming series will star Vicky McClure, Sheridan Smith, Martin Compston and Joe Cole.
The 40-year-old actor and TV presenter admitted she had 'nearly crumbled' from online abuse in the past.
The 39-year-old actor and TV presenter said she had received some 'shocking news' before deleting all her social media accounts.
The actor had a much-publicised struggle after her father was diagnosed with cancer back in 2016.
Smith feared it would harm her unborn child but learnt during while filming her documentary that mental health medication can often be taken by expectant mothers.
The 'Gavin & Stacey' star revealed she tried to quit her anxiety medication and was rushed to A&E after suffering seizures.
The actor said she 'jumped at the chance' to be a part of the series, which also stars Angela Griffin, David Threlfall and Robert Glenister.