In a blog post made today, The Witness creator Jonathan Blow revealed the voice cast for the game. The lineup includes Ashley Johnson (Ellie in The Last of Us) and Phil LaMarr (Hermes from Futurama among many others).

I would have a bit more on this, but it seems as though the website for The Witness is simply refusing to load right now at the time of this posting.

Fortunately, this blog was also posted to the PlayStation website.
Ashley Johnson
Ashley Johnson, who you may know as Ellie from The Last of Us, or from her roles in Tales from the Borderlands and Infamous: First Light. She also plays the smartest person on the hit TV show Blindspot. You can find Ashley on Twitter here.

Phil LaMarr
Phil LaMarr, who I best know as Marvin from Pulp Fiction, but who you may know as Vamp in Metal Gear Solid 2 and 4, or as Ratbag from Shadow of Mordor. He also played Hermes in Futurama and Ollie Williams in Family Guy! You can follow his random musings here.

Matthew Watersonhere.

Terra Devahere.

The Witness is due out in just under two weeks.