The correct way to ask this question is "How does Hardy depict ..." or "How does Sophy represent ..." the telling moments in Sophy's life. "Depict" means to draw, describe, delineate. These are functions reserved for the author and for the narrator that are not applicable to a character. Since there are subtle differences between the answers to these two questions, I'll explain how Sophy represents the telling moments in her life.
During the telling moments of Sophy's life--e.g., her rejection of Sam's affection on the evening of the death of the vicar's wife; her request to return to the vicar's employment; her decision to follow her son's "veto" instructions; her pitiful loss of her relationship with Sam--Sophy represents these by insisting upon doing what is right.
When Sam wants to kiss her in her parents' garden, she rebuffs him because it was not morally right to think of love immediately after a death. When She asks the vicar if she can continue in his service, she says the reason is that she and Sam had a quarrel. We don't know the nature of the quarrel, but since Sophy is neither bitter nor downcast, we can conclude that it was a quarrel over a point of honor or morality and that she insisted upon doing what was right. The same applies to the other two incidents listed above: Sophy acts as she does because she is convinced that it is the morally right thing to do.
One of the themes Hardy pursues throughout his corpus of work is the disadvantage women have in doing what appears to be right to them (sometimes this is a collective knowledge of what is morrally right, as in Sophy's case; sometimes this is an individual knowledge of what is right for the woman, as in Jude the Obscure). Thus Hardy shows the terrible disadvantages Sophy suffers from in the course of and as a result of doing what was socially morally right.
Her lameness became more confirmed as time went on, and she seldom or never left the house in the long southern thoroughfare, where she seemed to be pining her heart away. 'Why mayn't I say to Sam that I'll marry him? Why mayn't I?' she would murmur plaintively to herself when nobody was near.