the intent/meaning of the book and film

The supplmental material on the Blu-ray Disk includes reminiscences from Melvyn Douglas's granddaughter about the making of the film. It's obvious she has no idea whatever what the film is "about". The film's meaning is futher confused by Ashby's introduction of the "walking on water" scene, which is not in the book, and which he makes sure we "get" by having Chance thrust his umbrella into the lake.

It's unfortunate that the film, like Chance, has become a kind of inkblot onto which people project their own interpretations, when it should be obvious to any reasonably intelligent person what Kosinsky is getting at. The book doesn't "explain" the story, either  Kosinski had too much respect for the reader to do that  but it goes something like this:

Chance is a voyeur who has never actually experienced anything directly. Everything he knows is a vicarious experience from watching TV. And because his mind is filled with nothing other than what he sees on TV, he has never developed an "inner dialog" that might provide some framework into which he could place his "experiences".