It's funny the way Hollywood connects dots. Want to remake Godzilla? Get the guy who made the most memorable monster movie of the last few years. Need a new star for the next Planet of the Apes movie? Get the guy who had the most memorable monkey-related scene of 2012, Zero Dark Thirty's Jason Clarke.
Granted, the producers of Dawn of the Planet of the Apes were probably also quite impressed with the way Clarke delivered a character that was cold, calculating and charismatic all at once, but we're just going to assume it was his relationship with the ice cream-stealing monkey that pushed him to the top of their shortlist and won him the role. As for what that role is, 20th Century Fox isn't actually saying.
Mark Bomback's script is being kept in the shadows, though THR does disclose that the film will take place 15 years after Rise of the Planet of the Apes and it will focus on two different struggles, that of Caesar (Andy Serkis) trying to keep his simian uprising going and of a group of scientists living in San Francisco who are trying to save mankind. Presumably Clarke will be playing one of the scientists, but it's entirely possible that he could also be putting on a mo-cap suit as one of the apes challenging Caesar for his kingdom.
Either way we should know more about the cast once the Matt Reeves-directed film goes in front of cameras this coming spring.