Bronte uses juxtaposition to prove a point or make a comparison between characters. For example, Helen and Jane are juxtaposed in Chapter six:
I could not comprehend this doctrine of endurance; and still less could I understand or sympathise with the forbearance she expressed for her chastiser. Still I felt that Helen Burns considered things by a light invisible to my eyes. I suspected she might be right and I wrong; but I would not ponder the matter deeply; like Felix, I put it off to a more convenient season.
The juxtapostion serves to show how much more patient and forgiving Helen is than Jane.
Later, Mr Brocklehurst and Miss Temple are juxtaposed: Brocklehurst, who comes to Lowood looking like "the same black column which had frowned on me so ominously from the hearthrug of Gateshead" is contrasted with Miss Temple, who, under his influence, becomes "naturally pale as marble, [and] appeared to be assuming also the coldness and fixity of that material; especially her mouth, closed as if it would have required a sculptor’s chisel to open it, and her brow settled gradually into petrified severity." In this case, the juxtapostion shows Brocklehurst's effect on people: his rigid morality and frugality turns Miss Temple to stone.
The best example happens after the aborted marriage when Rochester takes Jane, the parson, and the lawyer to visit Bertha in her cell. Here, Rochester explicitly juxtaposes Jane and Bertha:
"Wood and Briggs, look at the difference! Compare these clear eyes with the red balls yonder— this face with that mask— this form with that bulk; then judge me, priest of the gospel and man of the law, and remember with what judgment ye judge ye shall be judged! Off with you now. I must shut up my prize.”
This is a comparison meant to show the fitness of Jane for marriage, compared to the woman Rochester is actually married to. But, if we think of Bertha as a double for Jane, it also can be seen to suggest the two sides of Jane's personality: one clear-eyed and rational, the other blood-shot and raging.
There are many more juxtapostions in the book; there is the vagrant Jane, starving in the cold outside Moor House, juxtaposed with Mary and Diana she sees studying in comfort inside the house; there is the juxtapostion of St John (another marble statue) with the lovely Rosamund; finally, the juxtaposition of the active, independent Jane with the blind, crippled Rochester at the end of the book. In each case, the pairings highlight crucial differences between characters.