A good way to think about the way this novel is narrated is to imagine that at any point, in any scene, the narrator is the smartest guy in the room. And not just the smartest guy, but a guy who wants to makes sure that everyone there knows he's the smartest guy. We get the sense that he knows everything about everyone, and that whenever he points his barbed finger of judgment at the stupid, awful, selfish people he's telling us about, we can only agree.
What's masterful is that we never feel the urge to rebel against Judgy McJudgerson. We really do feel that our narrator is right about whatever injustice he's describing. How does this happen? In part, it's because the level of the narrator's knowledge (his omniscience) tends to accommodate whatever scene we're watching. For example, check out the difference between the ways he condemns the monstrous Dorrit and the horrendous Circumlocution Office. Here is the scene where Arthur notices that Amy is wearing summer shoes in the winter:
Little Dorrit was not ashamed of her poor shoes. [...] Little Dorrit had a misgiving that [Arthur] might blame her father, if he saw them; that he might think, 'why did he dine to-day, and leave this little creature to the mercy of the cold stones!' She had no belief that it would have been a just reflection; she simply knew, by experience, that such delusions did sometimes present themselves to people. It was a part of her father's misfortunes that they did. (1.12.14)
And here is the passage where the narrator mocks how the Circumlocution Office defends its doings in Parliament:
Then would the noble lord, or right honourable gentleman, in whose department it was to defend the Circumlocution Office [tell the Parliament] that the Circumlocution Office not only was blameless in this matter, but was commendable in this matter, was extollable to the skies in this matter. Then would he be there to tell that honourable gentleman that, although the Circumlocution Office was invariably right and wholly right, it never was so right as in this matter. Then would he be there to tell that honourable gentleman that it would have been more to his honour, more to his credit, more to his good taste, more to his good sense, more to half the dictionary of commonplaces, if he had left the Circumlocution Office alone, and never approached this matter. [...] The Circumlocution Office was always voted immaculate by an accommodating majority. (1.10.7)
What do these passages have in common? There's a fancy analytical term for it: indirect discourse. This is when a third-person narrator uses the voice of a character to say something without putting quotation marks around it. So, for example, here we've got Amy's worrying how her dad comes off to Arthur. Who else but Amy would think that any blaming of Dorrit is "delusions" and just one of his "misfortunes"? This is certainly not the narrator's point of view; no, this is Amy's thoughts or speech patterns being included in the narration as indirect discourse. Same with the second passage: can't you just see the guy banging his fist on his podium and spouting his clichés about the Circumlocution Office being "blameless," "commendable," and "extollable"? Again, this is the imagined voice of the PR flack flimflamming his way to a favorable vote.
By impersonating the voices of the characters, the narrator deftly lets us know just how deeply he understands them, while at the same time firmly setting up his permanent and unyielding superiority over them. We see him as sensitive (he's able to describe the nuances of Amy's feelings) and so we trust him to make good judgments (e.g., condemning the government spin artist). How would these scenes be different if it were always just the narrator talking instead of the characters?