The character of Elizabeth is always doing the right thing.  Here she "instinctively turns away", because it is not right for her to show in emotion to or for Mr Darcy.  She must always be proper and not betray how she feels, but Austen is betraying her to the reader at the same time.  She is trying so hard to be proper and not appear to have put herself in his way, but the tension that is written in makes it feel like she might be trying to hard.

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