Tools of Characterization
Names are super significant in Dickens. Many times the actually words within the names have meaning. Frequently the names are portmanteau words – meaning they are combinations of two or more actual words that sound good in combination. One example is Mr. M'Choakumchild (get it? "choke-a-child"). He's not much of a character, but right away we know everything we need to know about him as a teacher.
Sometimes the names in the novel are onomatopoeic – meaning they sound like what they are describing. For example, there's Bitzer. There don't seem to be any words actually hiding in those syllables, like with M'Choakumchild, but still the name gives us clues into the character. Bitzer sounds sharp, short, and hostile. We can immediately feel all those things as soon as Dickens introduces him.
See if you can work out some other names: Gradgrind? Blackpool? Sleary? Scadgers? Are these names portmanteau words or examples of onomatopoeia?
It makes sense that in a novel that's deeply concerned with how to educate, a character's own education would be highly important. It helps, too, that it's an excellent way to show personality traits. For example, let's take the case of Bounderby. He is the self-made man at the most basic level – he has actually entirely made himself up. For most of the novel, we assume that his story of growing up a gutter orphan and being a good student at the school of hard knocks is accurate. It even helps to explain why he is the way he is. Sure, he's a loud, unpleasant, egotist, but as someone who really has achieved an enormous amount in his life, doesn't he have a right?
Of course, Bounderby's orphan myth is destroyed after Mrs. Pegler turns out to be his doting, loving mother. He was educated by his parents, who made the most of every opportunity and then apprenticed him at a young age (the best way to set up a young man in business back then). When his actual education path is revealed, Bounderby's existence turns out to be much more logical (not a meteoric rise to wealth and power, but a more normal one). Now he has no excuse for being a jerk all the time.
Imagine what happens when people are reading this novel for the first time. They read a little bit (a couple of chapters) in a magazine, and then they have to put it down for a week before they get the next section. They are reading four, five, maybe seven novels at the same time, since all of them are getting published this way, in short installments. For an author, one of the challenges is to remind readers what happened the last time so they don't feel lost, but to do it quickly so it's not annoying.
In TV shows nowadays, this is frequently done with an intro called "Previously On" made out of quick clips that highlight important things from before. Dickens made up another technique. Many of the characters are given some kind of prominent physical feature (either something in their appearance, or the way they move). This feature is frequently described pretty much every time the character appears, and usually with the same words, making an association that's easier to remember than just the character's name alone. So, for instance, Mrs. Sparsit is forever described in terms of her giant Roman nose, and every time we come across her, we remember, "oh, right, the nose lady." Same with Bitzer, who is always said to be very, very pale, almost translucently so. Can you find other examples?
Speech and Dialogue
The way people talk is a way of marking them, especially if their speech is different from the way the narrator and the rest of the characters sound. When we first come across Stephen and his northern, uneducated dialect, we immediately realize that he is a working-class man who has not been to school. The fact that Rachael has a similar dialect, even though it's less pronounced, marks her as a fit mate for Stephen, even if they can't actually be together. Another kind of class marking is Bounderby's insistence on using short names for everyone: he calls Louisa "Loo," because he is pretending to be a totally raw and uncultured guy… and because it demonstrates how high he really is in the grand scheme of social things.
Alternatively, some speech is used in the same way that a prominent physical feature is sometimes used – to make the reader remember the character. Mr. Sleary has a strong lisp and uses a lot of circus slang; Mrs. Gradgrind always dismisses unpleasant things with the phrase "I'll never hear the end of it"; and Slackbridge talks like a born orator, heavy on the "Oh my friends, oh my brothers!" exclamations.