Helen McCroy as Narcissa Malfoy in Harry Potter 6?

Well, this is a very interesting casting decision. Why not get Nicole Kidman or Naomi Watts? Narcissa is described as being beatiful, and Helen is kinda old and saggy looking.

Its way too small of a role for either Nicole Kidman or Naomi Watts. Plus there big name presence would overwhelm the movie. If you look at the casting of the films, they prefer to cast actors/actresses that are famous and respected, but more along the lines of semi-big names. Sure they are all famous but many people dont know who they are specifically, they just recognize them from other films. The actors cast in adult roles for Harry Potter have a strong theatrical background in Britain, just like Helen McCrory. I agree she doesnt exactly fit the description from the book, but for the most part most others dont either. I agree that is kind of weird casting, but she is a terrific actress and will fill the role amazingly I am sure. I wouldf have hated a big name actress like Kidman or Watts to have been cast in this role.
Did you know that McCrory was originally cast as Bellatrix Lestrange, the part that eventually went to Helena Bonam Carter, but had to drop out because of her pregnancy? I loved Carter as Bellatrix and envisioned her while reading it long before the movie came out, but isnt it weird that now McCrory is now cast as the role-she-originally-won's sister.

Actually, Narcissa is described as having beauty, but also having a face that's twisted in a way to makes it look as though she's constantly smelling something unpleasant. So, she's not that beautiful.

Helen is actually not that bad of a choice.