From Halopedia, the Halo wiki
Blindlight is the leading production services company at the nexus of Hollywood and the videogame industry. Their main services include writing, celebrity acquisition, casting and voice production. They use celebrities, voice-over specialists and unknowns to voice characters and help create more immersive and believable worlds in videogames.
They are responsible for casting the actors and producing the voice work for Halo 2, Halo 3 and Halo 3: ODST.