Well a fair while agao we had thought Paisha Coffey was going to be the model for Cortana in Halo 4 but the release of the A Hero Awakens video clearly shows that Mackenzie Mason has done the cinematics modelling / body acting / performance capture for the character of Cortana.
This role means Mackenzie got to run around and interact with the other actors, and most obviously play the Master Chief's counterpart. Her movements are the movements of Cortana in game.
|Mason doing the performance capture for Cortana.|
The voice of Cortana will continue to be voiced by Jen Taylor who has done the voice over for every Halo game.