Update: The movie put out casting calls last month for several of the main characters:
The casting calls appear to be from a draft of the script, written by Star Trek's Roberto Orci and Alex Kurtzman, and they give a pretty good idea of just how closely this film will stick to the book's story.
Here's a description of the movie Ender:
He's depicted as smart and sensitive, but also incredibly ruthless. And he's ten years old — older than in the book, but not as old as Hollywood was trying to make him at one point. There are a few scenes where he worries about being like his cut-throat brother Peter, and confides in his sister Valentine. Just like in the book, he dishes out a rough treatment to Bonzo Madrid, his former platoon leader, when Bonzo tries to bully him too much. And then he feels bad about it. The screenplay also includes some scenes where Ender has weird nightmares about the buggers — and he tries to understand where the buggers are coming from, and what their children are like. Ender is pissed at Graff because he keeps changing the rules in the war "exercises."
!The scene where Ender finds out that his final victory was not, in fact, a game is pretty intense, and features Ender and Graff both trying to talk at the same time. Ender is saying "They came to establish a colony, we chased them away... in fifty years they have never returned," while Graff is saying, "It makes no difference now," and then Ender is saying "Now I am become death, the destroyer of worlds," at the same time as Graff is saying "What are you talking about?" Finally, Ender is saying "I will bear the shame of this xenocide forever," at the same moment as Graff is saying, "You will be remembered as a hero." It ends with Ender getting an injection, knocking him out.