Naomi Watts for Harry Potter and the Half-Blood Prince
To figure out what role Watts might be playing, one only has to read the first couple chapters of the novel. Who do we meet? Narcissa Malfoy, of course! The blonde mother of Draco Malfoy makes a visit to Severus Snape early in the book to make a plea for her son. If you haven't read the book, then you don't deserve to know what she was pleading for, but it is very, very important.
Helena Bonham Carter as Bellatrix Lestrange and Naomi Watts as Narcissa? Don't get me wrong, but it sounds like the Death Eaters know how to choose their women.
We see Narcissa a few more times in Half-Blood Prince and then some in Deathly Hallows. Unlike Bellatrix, there are few moments where we actually see Narcissa pull out her wand.