Source Material: Philip K. Dick’s Do Androids Dream of Electric Sheep? (1968)
In the Movie: Deckard is a former cop/android hunter who is convinced to take one last job to “retire” a group of rogue Nexus-6 replicants. Several of the Nexus-6s are passing as human, but one usually reliable way of identifying an Andy is using the Voigt-Kampff test, which measures a person’s ability to feel empathy — something the androids do not possess. It is eventually revealed that implanted memories can result in an android who doesn’t know it’s an android, something Deckard discovers after testing Dr. Eldon Tyrell’s assistant Rachael (Sean Young), an unwitting replicant herself.
The replicants are pre-programmed with a limited lifespan, and so the group’s leader, Batty (Rutger Hauer), wants to confront his creator Tyrell in hopes of finding a way to get “more life.” In pursuit of that, Batty and “basic pleasure model” Pris (Daryl Hannah) befriend kind-hearted replicant engineer J.F. Sebastian (William Sanderson), whom they manipulate into taking Batty to see Tyrell, a meeting which does not end well for Tyrell.
During his hunt for the replicants, it is strongly implied that Deckard himself may not be human, and thus may be hunting and killing his own kind. He also falls in love with Rachael. The film ends with Deckard and Rachael going on the run, hoping to make a life for however much time they have left.
But in the Book: Deckard is a bounty hunter tasked with hunting down renegade androids (“andys,” for short) in 1992 San Francisco. Earth is a thoroughly depressing place, having been ravaged by a world war that drove much of humanity to offworld colonies and wiped out many of the native animal species. As in the movie, Deckard must track and kill a group of rogue androids advanced enough to pass for human, with some even possessing artificial memories and no knowledge of true nature. The book also follows John Isidore, a simple-minded bloke who helps the androids (and who essentially became Sebastian in the film).
As in the film, artificial animals are prevalent, but they are far more important in the book. Since biological animals are mostly extinct, owning one is a huge status symbol, and one of Deckard’s primary motives throughout the book is to acquire such an animal. (He used to own a real sheep, but after it died he bought an artificial one — hence the title.) In keeping with the book’s themes of empathy, Dick’s story also details “Mercerism,” a new religion which uses “Empathy Boxes” to link users into, as Wikipedia puts it, “the suffering of Wilbur Mercer, a man who takes an endless walk up a mountain while stones are thrown at him, the pain of which the users share.”
Both the book and film brilliantly explore notions of free will, what constitutes humanity, and the ethics of creating what is essentially a slave race — a nice contrast to the idea of the androids being the ones incapable of empathy.
One last fun factoid: in the book Rachael and Pris are both the same model, so if the movie had remained faithful either Sean Young or Daryl Hannah would have played a dual role.