Natalie Portman was chosen to play Evey from a deck of possibilities like Bryce dallas (We will remember her from films like The Young Woman from the Water, The Forest or the recent Spider Man 3 in the role of Gwen Stacy) or Scarlett Johansson (currently news for being Woody Allen's muse).
Currently the actress has more projects, already in post-production such as The Darjeeling Limited, Mr. Magorium's Wonder Emporium o The Other Boleyn Girl.