This one's a little tricky. For the most part, we have a narrator who knows everything there is to know about what's happening with Stanley, but not much else. Although in some sense we don't get as much information as we might expect about Stanley's feelings (check out the "Tone" and "Writing Style" sections for more on this), we definitely see things from inside Stanley's head and hear a lot about how he's experiencing the world. The other characters only appear to us as Stanley sees them. We never know what Zero is thinking, for instance – only what he does, or what he tells Stanley.
This is a great way to build sympathy for Stanley, particularly because Stanley starts out as such an isolated character. He doesn't really confide in anyone (or talk to anyone, for that matter), and he's not exactly what you'd call a man of action. Hearing his thoughts and seeing the world through his eyes may be the only way that we can really get to know him.
Even in the Kate Barlow and Elya Yelnats parts of the story, the narrative perspective remains pretty much the same, just with different characters. We hear about the events of the stories as Kate and Elya experience them – Stanley wasn't even born then, after all – and we share their thoughts as we go along.
It doesn't end there, folks. At certain points in the story – at the very beginning, when Sam is murdered, and in the last chapter, to name a few – the narrator's view becomes much less limited. It's almost as if he's suddenly looking down from a great height, able to travel across large stretches of time and zero in on tiny details all at once. (We want to know how we can get in on this deal, by the way.)
And bonus: when he's using this Super Omniscient voice, the narrator often steps out of the story and addresses the reader directly. Sometimes this takes the form of second-person voice: as in, "you don't want to be bitten by a yellow-spotted lizard" (1.12). Other times, he just talks about us as "the reader": as in, "the reader probably still has some questions" (50.3). The most stand-out instance of this comes after Sam's death, where we find the following passage:
That all happened one hundred and ten years ago. Since then, not one drop of rain has fallen on Green Lake.
You make the decision: Whom did God punish? (26.43-44)
Whoa, now. In a book whose narrator is generally un-intrusive, this is a pretty startling moment, don't you think? How does the change of voice affect how you feel or think about what happened to Sam? Is there something that the narrator accomplishes by addressing the reader directly that he couldn't have done if he had used a different approach?