what is empathy in this book? And what characters express empathy and what characters dont?

Empathy is the main theme of the novel and is the crux on which Dick's metaphysical reflection on the meaning of life hangs. Each character in the novel must deal with what it means to be empathetic and whether that allows someone to be valued as a living thing. Rick hates his electric sheep precisely because he believes it cannot feel any love for him, even though he cares for it. This feeling allows Rick to perform his work as a bounty hunter because he believes that androids, like his sheep, are incapable of true human emotion and therefore not worthy of life in a society in which life is the highest ideal. Rick notes early on that herbivores or omnivores are the only creatures with the empathetic impulse and that empathy is what allows humanity to survive.

Yet, Rick soon learns that androids may be capable of empathy and humans may be able to be devoid of empathy; this in turn causes a extreme shift in Rick's understanding of himself. Suddenly, Rick finds that the lines between what one can call living or what one can call not-living are blurred. Androids find their empathetic abilities with each other just as humans find the ability to be empathetic in a collective group. Humans, also, are capable of a loss of empathy. This is demonstrated through the character of Phil Resch who, Rick finds, enjoys killing simply for killing's sake.