They've already casted Bellatrix Lestrange (Helen McCrory), but I think I could've done a better job. There aren't too many descriptions of Bellatrix in the HP books, but I found two pretty good quotes on her appearance:
"Azkaban had hollowed Bellatrix Lestrange's face, making it gaunt and skull-like, but it was alive with a feverish, fanatical glow."(Order of the Phoenix, 783)
and
"Dark as her sister was fair, with heavily lidded eyes and a strong jaw, she did not take her gaze from Snape..."(Half-Blood Prince, 23)
Who would I cast? Wait for it, wait for it...
Cher! If she doesn't scream heavily lidded eyes and gaunt bone structure, I don't know who does.