Why not extract a quotation from the play and explain how it is central to characterization or theme? For instance, Cassius tells Brutus in Act I, Scene 2,
Men at some time are masters of their fates:
The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings. (1.2.145-147)
Write a thesis, then, on how these words hold true for Brutus and Cassius or Caesar and Brutus, comparing and contrasting two characters. Or, write a thesis on how these lines illustrate the theme Fate vs. Choice and explain by analyzing certain actions of certain characters that affects their lives.