We don't get a ton of direct characterization here, where the narrator basically tells you all about the character. What we do get, though, is generally quite pithy and quite funny. Catherine gets the most direct characterization, most of it related to how she's better looking than she used to be. Other characters, like Henry, get a very brief rundown of age and basic manners as a form of introduction. Austen does most of her character building work elsewhere, but the little details she provides us upon introducing a new character are often extremely important. For instance, knowing that Isabella is four years older than Catherine accounts for some of her behavior and her desire to get married. If Catherine is seventeen, that makes Isabella twenty-one. And 21 was actually considered a bit old to be unmarried in this period. Isabella would be feeling the pressure to get hitched, and soon.
The old maxim 'actions speak louder than words' may not really apply to Northanger Abbey, where speech and language are quite important. But actions certainly speak along with words in this text. Catherine's good opinion of Henry is further cemented when she contrasts his actions, like his carriage driving skills, to John Thorpe's. The borderline wild and scandalous actions of the Thorpe siblings, like their unchaperoned road trip to a castle, also contrasts to the more refined and restrained actions of the Tilneys, who enjoy going on nature walks. Actions often help to reveal the true nature of a character in this text as well. Isabella's blatant flirtation with Captain Tilney reveals her shallow and flighty nature to the Morlands. General Tilney in particular is 'outed' by his actions, so to speak. Though he puts on a good show, he is revealed to be a snobby and rude after he throws Catherine out of his house.
Social status matters in the world of Northanger Abbey, possibly even more than in a high school. Status and class determine just about everything here: how people interact, what they do, and how they behave. Of course, one social class doesn't have a monopoly on good or bad behavior in this book. Captain Tilney and Isabella both behave scandalously, despite their differences in income. But Captain Tilney probably wouldn't be such a scoundrel if he didn't have the money to get away with it. And Isabella wouldn't be such a relentless flirt if she weren't trying to find a rich man to marry.
Why are thoughts and opinions listed up there, too? Well, the way people think about the social status of others tells us a lot of about who they are. Social status largely influences people's opinions. The wealthy General Tilney is prejudiced against those with lower incomes. Catherine's own kindness is revealed by her sentiments on marrying for money, of which she disapproves.
Austen definitely lets her characters speak for themselves here. Northanger Abbey has a lot of dialogue and each character is largely defined by how they speak and what they say. There's probably no better indicator of the contrast between John and Henry than their dialogue, for instance. John's shouting and swearing and contradictions all contrast with Henry's witty and charming speech. Austen rarely just tells us about her characters. She let's us learn about them through their dialogue. And dialogue often reveals things about characters that the narrator fails to tell us, or that Catherine fails to understand. For example, though Catherine fails to make note of it, we learn to be suspicious of Isabella from her sly and convenient responses to Catherine. Dialogue reveals who is funny, who is inherently decent, and who is duplicitous here.