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)