When you write an extended literary essay, often one requiring research, you are essentially making an argument. You are arguing that your perspective-an interpretation, an evaluative judgment, or a critical evaluation-is a valid one. Like any argument paper you have ever written for a first-year composition course, you must have a specific, detailed thesis statement that reveals your perspective, and, like any good argument, your perspective must be one which is debatable.
Examples:
You would not want to make an argument of this sort:
Shakespeare's Hamlet is a play about a young man who seeks revenge.
That doesn't say anything-it's basically just a summary and is hardly debatable.
A better thesis would be this:
Hamlet experiences internal conflict because he is in love with his mother.
That is debatable, controversial even. The rest of a paper with this argument as its thesis will be an attempt to show, using specific examples from the text and evidence from scholars, (1) how Hamlet is in love with his mother, (2) why he's in love with her, and (3) what implications there are for reading the play in this manner.
You also want to avoid a thesis statement like this:
Spirituality means different things to different people. King Lear, The Book of Romans, and Zen and the Art of Motorcycle Maintenance each view the spirit differently.
Again, that says nothing that's not already self-evident. Why bother writing a paper about that? You're not writing an essay to list works that have nothing in common other than a general topic like "spirituality." You want to find certain works or authors that, while they may have several differences, do have some specific, unifying point. That point is your thesis.
A better thesis would be this:
Lear, Romans, and Zen each view the soul as the center of human personality.
Then you prove it, using examples from the texts that show that the soul is the center of personality.