In Great Expectations, considerable humor is derived from Dickens' frank descriptions of his colorful cast of characters. Uncle Pumblechook, who's about as colorful as it gets, is given to us as a "large, hard-breathing, middle-aged, slow man, with a mouth like a fish, dull staring eyes, and sandy hair standing upright on his head." As for Mr. Wopsle, he's described as a man who has "united to a Roman nose and a large shining bald forehead".
As well as being humorous, such descriptions tell us an awful lot about what kind of people we're dealing with. Here as elsewhere in Dickens, what you see is what you get.
Dickens mainly uses pathos to highlight the vulnerability of his characters. The most obvious example in Great Expectations would be the scene in the graveyard, when young Pip first encounters Abel Magwitch. Pip's young age, combined with the fact that we know him to be an orphan, make him especially vulnerable. Add in a confrontation with an escaped convict who threatens to kill him and eat certain of his inner organs, and you generate a considerable amount of pathos.
Later on in the story, it will be Magwitch's turn to be vulnerable, as he desperately tries to make good his escape from England with the assistance of Pip and Herbert Pocket. Once again, Dickens uses pathos to generate sympathy for those who, for one reason or another, find themselves isolated in society.