Although a thesis is the main point of an essay or analysis, most people don't know, when they being writing, what their thesis will be. Their main point usually emerges to them after they have written for a while.
To come up with a good, strong thesis, I suggest you write a "discovery draft." This means you spend some time writing freely about different aspects of the question, not worrying yet about whether what you are producing is coherent or would make a good final product. This discovery draft is for your eyes only, but it will help you discover your thesis.
First of all, I suggest you spend about 15 or 20 minutes—more if inspiration strikes you—writing about your reaction to the play. What scenes stood out most to you? Which characters did you most resonate with—or recoil from? What felt powerful, what felt unfinished? Did anything bother you about the play? These are the types of questions that will help you determine what your reaction was to the play. By the end of this exercise, you should have an emerging sense of "your understanding of the play."
After you do your discovery draft, set it aside and take a break. Then come back and do a second discovery draft. This one should be about the sentence, "The central issue of the play is the conflict between duty and desire." In this discovery draft, try to prove that this is true. Does Macbeth represent desire, and someone else (MacDuff? Banquo?) represent duty? Or is Macbeth duty, Lady Macbeth desire? Or do you find duty and desire both at war within the various characters? Once again, spend about 15 or 20 minutes writing about this. Don't worry if you don't reach a firm conclusion.
After this, set both discovery drafts aside and take a break again, ideally overnight.
It's possible that, because you have spent some time thinking about your own reaction to the play and about duty/desire, your mind will spontaneously start to answer the assigned question and come up with a thesis statement. If so, that's great. Jot it down while it's hot; then you'll have it on hand when you buckle down to writing your paper.
If that doesn't happen, don't worry, as you still have your discovery drafts. After taking your break, get them both out and look at them. Now you know your reaction to the play, and you have some thoughts on duty/desire. From these, you can form a thesis that answers the question.
It might be along these lines:
- "The conflict between duty and desire is very close to my understanding of the play, because [insert insights from duty/desire writing, combined with your reaction]." OR...
- "I have a very different understanding of what the play is about. It's not about the conflict between duty and desire so much as it is about [insert material from your reactions to the play, planning to show why this is more important than duty vs. desire]." OR...
- "While duty and desire are certainly strong themes in Macbeth, I had trouble seeing them as the main theme because they were overshadowed by [insert overwhelming thing from your reaction discovery draft]."
Once you have done this, you will end up with a thesis statement that really reflects what you think and feel about the play. Once you have that statement, it will be much easier to write the rest of the analysis. The thesis statement, and the prewriting that you've already done, will guide you. You will probably end up using ideas from your discovery drafts, though you probably will not use the very same sentences.
Writing a discovery draft might seem to take more time up front, but it saves time in the long run because it makes the writing process much easier.