Thackeray's narrator usually functions as our worldly and semi-jaded guide through the social world of the novel. He has been there, done that, and come back to show us the T-shirt. Through a combination of universal statements, little cutting asides, and a constant feeling of condescension, the narrator's voice gains an authority with the reader that's hard to shake off or question. Let's check out how this works in a little section from the novel's first chapter.
Although schoolmistresses' letters are to be trusted no more nor less than churchyard epitaphs; yet [...] in academies of the male and female sex it occurs every now and then that the pupil is fully worthy of the praises bestowed by the disinterested instructor. Now, Miss Amelia Sedley was a young lady of this singular species; and deserved not only all that Miss Pinkerton said in her praise, but had many charming qualities which that pompous old Minerva of a woman could not see [...]
[Amelia] had twelve intimate and bosom friends out of the twenty-four young ladies. Even envious Miss Briggs never spoke ill of her; high and mighty Miss Saltire (Lord Dexter's granddaughter) allowed that her figure was genteel [...]
But [Amelia] is not a heroine, there is no need to describe her person; indeed I am afraid that her nose was rather short than otherwise, and her cheeks a great deal too round and red for a heroine; [...] when the day of departure came, between her two customs of laughing and crying, Miss Sedley was greatly puzzled how to act. (1.24-27)
First, we come across a generalization that has the undisputable air of a wise and ancient proverb: "schoolmistresses' letters are to be trusted no more nor less than churchyard epitaphs." This is written with an easy confidence, and we readers are supposed to react the way we would to any proverb: "Oh, yes, of course, how true – recommendation letters and gravestones do tend to be overly positive."
Next the narrator turns to give us the real scoop on this Amelia character. She turns out to be "fully worthy of the praises" of Miss Pinkerton. How do we know the narrator is telling the truth? Because he's not the kind of person who compliments without merit. Miss Pinkerton, for instance, he calls a "pompous old Minerva," showing us that he can call it like he sees it at all times.
Next we get a sense of exactly the kind of people Amelia goes to school with and what kind of place Miss Pinkerton's Academy is. Pay attention to the way the narrator gives us a brief and crystallizing glimpse of Miss Saltire. What does she think is her most important quality? That she is the granddaughter of a Lord. From the parenthesis, we get a sense that she insists this fact about her social status has to follow any introduction of her own name. This is what gives her the ability to judge Amelia's figure as "genteel" (a.k.a. befitting a member of the aristocracy).
And finally there is a rebuff to the reader. Oh, all this time you were thinking I was telling you about the heroine of this work? Think again, you fools. Amelia "is not a heroine." And then, just when the narrator has been telling us how great Amelia is, he goes all negative on her. She has some appearance deficiencies and also is kind of an idiot, whose only states are "laughing and crying." The last few sentences of the quotation demonstrate that the narrator expects the reader not to form any impressions or opinions without his say-so. Don't you worry your pretty little heads trying to figure things out, the narrator implies, I'll tell you what to think and when to think it from here on out.