I didn't find a case study per se, but I did find a very descriptive synopsis of the book on Wikipedia. *See http://www.en.wikipedia/wiki/Sula_(novel) for the full synopsis.* Here's a few lines from it that might help you see what her character did to develop the plot and story line:
. . . the town regards Sula as the very personification of evil for her blatant disregard of social conventions . . . Their hatred in part rests upon Sula's interracial relationships . . . Ironically, the community's labeling of Sula as evil actually improves their own lives. Her presence in the community gives them the impetus to live harmoniously with one another. . . Just before Sula dies in 1940, they achieve a half-hearted reconciliation. With Sula's death, the harmony that had reigned in the town quickly dissolves.