EEG Team assembling the Voight-Kampff machine based on Syd Mead’s sketches.
Why did Holden need to VK Leon, if the police knew what he looked like? This test is more crucial in the novel, where it is intimated that there are humans who have actually been replaced by look-alike replicants. Replicants, however, can readily change their appearances and aren’t easily recognized from photographs, e.g., Zhora’s tattoo, and Pris’ raccoon makeup. In the July 1980 screenplay, Deckard muses, “They could change their appearances but not their future.” In the December 1980 screenplay, Deckard says Zhora’s ”black hair is a wig which now hangs on the wall next to the shower. She didn’t look like Nexus designated Zhora to begin with, but even less now.” » Blade Runner FAQ
The Voight-Kampff empathy test is probably as valid a test as there is ever going to be - because it is testing for something beyond intelligence, but which is still a form of intelligence. It is sort of a higher form of intelligence, that is, a concern for other living beings. What the test really asks the replicant to demonstrate is a reciprocal concern for other life. One is concerned for the replicants and one asks in return from the replicants a concern for the lives of other creatures. The replicants are entitled to this concern but only if they themselves can exibit it. (Philip K. Dick - Souvenir Magazine 1982)