I read, that Josh Brolin first said "no" to the film, but then properly read the script. Since it wasn´t overly political film (or just "Bush-bashing"), Brolin then agreed. You may or may not like the man, but taking the "Michael Moore approach" with this film would´ve probably been disaster. I think that with every biopic (or related to "biopic"), you kinda need to "humanize" the main character(s).
Stone in Wikipedia:
"I want a fair, true portrait of the man. How did Bush go from an alcoholic bum to the most powerful figure in the world? It's like Frank Capra territory on one hand, but I'll also cover the demons in his private life, his bouts with his dad and his conversion to Christianity, which explains a lot of where he is coming from. It includes his belief that God personally chose him to be president of the United States, and his coming into his own with the stunning, preemptive attack on Iraq. It will contain surprises for Bush supporters and his detractors."