The 31-year-old actress, known best for roles in Two Pints of Lager, Benidorm and Gavin & Stacey, believes ITV's Great Train Robbery drama provides her with a more "sophisticated" part as Charmian Biggs, the wife of Ronnie.
She told What's on TV: "It's really grown-up for me. I don't usually get to play sophisticated women, I usually play the scruffs, the chavs and the slappers.
"Although I've loved the roles I've played, it's been so nice they've given me this opportunity and it's scary, you feel a bit out your comfort zone, but I'm loving every minute."
Smith recently confessed that filming the five-part series made her feel broody.
Mrs Biggs continues on Wednesday (September 12) on ITV1.