On playing Nessarose...

To me, Nessarose is quite the determined individual - she doesn't want to be bound by the rules of society, the rules that effectively tell her that she's useless because she's in a wheelchair.
She's desperate for acceptance, but she's not sure how to make herself accepted. She might be "tragically beautiful", but that doesn't count for much in Oz.
Boq is a ray of light in her life, she adores him to an almost obsessive level... and his 'betrayal' breaks her heart, makes her vengeful and wrathful.

What makes her the Wicked Witch of the East? Frustration.
In the book, fifteen years passes between Elphaba leaving in the Emerald City and her finally meeting her sister again. In the musical, the delay is shortened to but two years, but still...
In those two years, she keeps a hold over Boq, but he never loves her, never calls her 'Nessarose' instead of 'Madame Governor'. In those two years, she hears stories of her sister flying around Oz and saving Animals that she's never even met... and not even having a passing thought for her dear wheelchair-bound sister.
This, inevitably, frustrates her. It's not surprising she lashes out at Elphaba and Boq during the 'Wicked Witch of the East' segment of the show.

And I know this sounds strange, but... I think that whole incident changes her for the better.
When she's hit by the house, I'm a firm believer that she was talking to the Munchkins... and that she was about to tell them that they were free.
Nessarose isn't heartless, after all, and... I think hearing Boq's protests, and then losing him, would make her want to set 'her people' free.

Of course, not many people see Nessarose as redeemable at all. I think that's a shame, really...