In Macbeth, it is Macbeth himself who points out in Act I, scene iii, line 141, that "Nothing is but what is not." He is highlighting his own confusion and the fact that, having been told by the witches that he will, first of all, be Thane of Cawdor and, secondly, king, before even Duncan's men have a chance to share the news of his new title as Thane of Cawdor, the only possible reality for him at that moment is what is not real. He therefore becomes obsessed with the need to be king and soon forgets that perhaps he can become king "without my stir" (143), meaning without any scheming and dishonesty. When this is considered with the likelihood that "Fair is foul and foul is fair," from Act I.i.10, the audience must prepare itself for many changes.
Presumably, this essay will cover changes in various characters and the extent to which those changes contribute to the plot and development of the story. The introductory paragraph needs to attract the reader's attention and make him or her sufficiently interested to read on and actually become involved, in this instance, with the characters. The thesis statement, which will appear in the introductory paragraph, basically states the writer's purpose and so must be general enough to outline the essay. Finally, the introductory paragraph must make the transition to the first body paragraph smoothly by including a "hook" which ensures that the essay flows. A possibility to consider is:
William Shakespeare's Macbeth is full of intrigue and deceit. It is the characters who drive the plot forward as they manipulate and shape circumstances to fit their own agendas: (Thesis). The witches will transform from spell-making sisters, strange and cunning, into the source of Macbeth's confusion. They will become for him the driving force behind his own transformation from noble and decorated soldier to murderous villain. Add to this Lady Macbeth's contribution to Macbeth's obsession and her own altered identity as she changes from a woman unnerved by her own "direst cruelty" (I.v.40) into a delusional and guilt-ridden person, obsessed only with the "damned spot" (V.i.33) which plagues her conscience. It is the changes in these characters which bring about the devastating end to the play: (hook).
You can then proceed, in the body paragraphs, to elaborate on the characters, their specific changes and their influences on the outcome of Macbeth.