The Gavin & Stacey comic, who is currently starring in a production of Legally Blonde, appears on the reality series tonight as a mentor to a selection of the final 20 contestants.
Smith, Spice Girl Mel C, OIiver! actress Jodie Prenger, Tamzin Outhwaite, Kelly Ellis and Ruthie Henshall offer advice to the West End hopefuls, who are aiming to be chosen for the role of Dorothy in Andrew Lloyd Webber's new production.
Contestants Katie Honan, Bronte Barbe, Claire Harbourne and Lauren Samuels all impress Smith, with one of the girls moving her to tears after singing the title ballad.
Speaking afterwards, Smith says: "This is such an honour for me, I'm the world's biggest Wizard of Oz fan. That was just brilliant girls. That's how it should be done. I should take tips from you!"
Over The Rainbow continues tonight at 6.30pm on BBC One.