To successfully write on this topic, you’re going to need to show some specific examples of how prejudice affects the judgment of several of the jurors in Twelve Angry Men. Although many of the jurors exhibit some prejudicial behavior, the most biased jurors are juror #3 and juror #10. Think about what these jurors say that exposes their judgment and shows that they are making decisions based on beliefs that are not related to the crime in question.
As far as an introduction is concerned, you need to be general in how you begin things. Don’t jump right into giving examples of what the jurors have said or done; save that for the body of your essay. You should start off saying something broader about prejudice and how it affects many of us, often without our own knowledge. Also say something about how the play Twelve Angry Men shows that prejudice can control people’s thinking without their awareness of it. Then you’ll be ready to move into specific incidents from the play.