I think that the starting point for this essay would be to develop a statement that links all the side characters together. It might help to stress in the introduction that part of the drama's strength is precisely that its thematic relevance is evident in all of the characters and not merely the primary ones. For example, the resistance that Proctor shows in the middle and end of the drama is reflected in characters like Francis Nurse and, most notably, in Giles Corey. Another instance works the other way with making the point that the manipulation of people in order to consolidate one's own power and political purpose seen in Abigail is also present in Danforth and Hathorne or the Putnams. This is an approach that one can take in the construction of such an essay. It becomes important to be able to explain how the major characters are thematic representatives as much as the main characters. Another approach that can be taken is to make the point that the hysteria and true horror of the Salem Witch Trials as seen in Miller's work is that it was not limited to one or two people, but rather was a contagion that impacted nearly everyone in the town. This enables you to almost bypass Proctor and Abby and focus on the other characters to bolster the thesis.