Today's Guest: Katie McGrath, actress, 'Merlin,' 'Supergirl' Katie McGrath plays Lady Morgana in the new NBC dramatic import, “Merlin.” The shows airs every Sunday at 8 p.m. Katie is a relative newcomer to acting, as I understand it, although American audiences may have seen her in an episode of Showtime’s popular drama, “The Tudors,” before she landed a plum role on “Merlin.” Did I say plum? That’s not quite accurate, because she’s more of a peach! KATIE McGRATH podcast excerpt: 'If I could…