This pretty much sums it up:
What Keaton brings to his characterization of both Batman and his millionaire-playboy alter ego, Bruce Wayne, is a quality of coiled concentration, a wary vigilance. In his Batsuit, Keaton's movements are stylized, almost robotic, and the stiffness of movement carries Arthurian associations, as if he were indeed a dark knight, armored for battle
But as evocative as he is in his Bat regalia, it's as Bruce Wayne that Keaton announces his own arrival. This is a true star performance, subtle, authoritative and sexually vibrant.there's genuine pain in the performance, signs of a wounded man trying to shake free of childhood traumas.The Warren Skaaren-Sam Hamm script portrays Wayne as a realist who isn't sure himself why he does what he does. Driven by the vision of his parents' murder, his life is not his own. (Washington post 1989)