A priestess of R’hllor from the fabled city of Asshai beside the Shadow, Melisandre is known as “the red woman”: she wears red robes and gowns, her hair is a deep burnished copper, even her eyes are red. Taller than many knights, her appearance is strikingly beautiful, slender, with a heart-shaped face, a full bosom, and a narrow waist. She converted Queen Selyse to the faith of R’hllor, and has gained great influence at Stannis’ court.
Carice van Houten is not so tall, but she’s certainly striking in her appearance. Given the trouble actors on the show have had with colored contacts, it’s unclear whether she will sport Melisandre’s red eyes.