Three words come up again and again in ch.43: astonishment/surprise, civility/politeness, and alteration:

The housekeeper came; a respectable-looking elderly woman, much less fine, and more civil, than [Elizabeth] had any notion of finding her. (a reflection on Mr. Darcy?)

[Elizabeth] listened with increasing astonishment as the housekeeper added, "I have never had a cross word from him in my life."

[Darcy] spoke to Elizabeth, if not in terms of perfect composure, at least of perfect civility.

[Elizabeth] knew not what answer she returned to [Darcy's] civil enquiries after her family. Amazed at the alteration in his manner since they last parted...

And his behaviour, so strikingly altered -- what could it mean? That he should even speak to her was amazing! -- but to speak with such civility, to inquire after her family! Never in her life had she seen his manners so little dignified, never had he spoken with such gentleness as on this unexpected meeting. What a contrast did it offer to his last address in Rosings Park, when he put his letter into her hand! She knew not what to think, nor how to account for it.

Perhaps he had been civil only because he felt himself at ease...

Elizabeth's astonishment was quite equal to what it had been at first, by the sight of Mr. Darcy approaching them (as they walked through the park).

With a glance, she saw, that he had lost none of his recent civility.

Mrs. Gardiner was standing a little behind; and on [Elizabeth] pausing, he asked her if she would do him the honour of introducing him to her friends. This was a stroke of civility for which she was quite unprepared.

The conversation soon turned upon fishing; and she heard Mr. Darcy invite [Mr. Gardiner], with the greatest civility, to fish there as often as he chose while he continued in the neighbourhood...Mrs. Gardiner, who was walking arm-in-arm with Elizabeth, gave her a look expressive of her wonder.

[Elizabeth's] astonishment, however, was extreme, and continually was she repeating, "Why is he so altered?"

The surprise of such an application was great indeed (of Darcy asking to introduce Georgiana to Elizabeth).

They parted on each side with the utmost politeness.

The observations of her uncle and aunt now began; and each of them pronounced him to be infinitely superior to anything they had expected. "He is perfectly well-behaved, polite, and unassuming," said her uncle...I was never more surprised than by his behaviour to us. It was more than civil; it was really attentive."

[Elizabeth] had never seen him so pleasant as this morning.

[Elizabeth} could do nothing but think, and think with wonder, of Mr. Darcy's civility.