Cortana actress halo show: 59 Photos
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According to Wolfkill, as planning for Season 2 began, Halo's creative team wanted to be able to focus on the facial fidelity of Cortana moving forward. The original Season 1 Cortana was a fully CG character. It was not based on an actor.
The linked article indicates that changes to Cortana's appearance were made to appeal to long time fans who were displeased with the physical portrayal by Jen Taylor in Season 1. Taylor still voices Cortana, but the face (now offered by Christina Bennington) is entirely different.
According to executive producer Kiki Wolfkill, who was promoted to showrunner for Halo Season 2, Cortana's Season 1 design was changed to capture the full range of emotion and facial nuance that only a real actor can bring (via GameSpot): The original Season 1 Cortana was a fully CG character.
Cortana's new look is down to there being a new showrunner in charge of Halo's second season - David Wiener.
Despite the changes to her design, Taylor still voices the character in Season 2. She was involved with the motion capture of Cortana in the previous season. But those duties have been taken over by London's West End star, Christina Bennington.