the "unknown" actress
I do agree with the posts below in regards to Swank being a good choice because no one could identify her with any previous roles. I was also thinking of other reasons it would be beneficial to use Swank and I came up with the idea that it was possible that Swank might be bolder and able to do a more controversial role because she was new to the Hollywood scene.
Many actresses have in their contracts what they will and will not do with regards to nudity and sex scenes. Swank probably wanted to make a big impression (I assume) in the Hollywood scene and probably would not object to the demands that the movie had. Because of this, Swank was a good choice because she might have been more willing to do controversial nudity and sex scenes that might not have been done by using another actress. It is probably true that well known actresses would have not had a problem with the nudity or sex scenes, as in the case of the film "Monster" but everyone knew what Charlize Theron looked like and for me at least during most of the movie I was wondering how she looked so different. Because I had never scene Swank before the thought of what she really looked like never occurred to me. Because of the combination of her probably not having many restrictions on her contract and because she was new to the scene made Swank the perfect choice for the film because these attributes made her believable.