I think in addition to reference to the meaning of the crucible in the response above, you might want to think about the way in which that our characters and identities are never really shown in their full glory or truth in times of ease. It is relatively easy for someone to be good and truthful when they are not facing any pressure or when their is no price to that goodness. It is a completely different matter indeed when characters face situations like that of John Proctor, Giles Corey and Rebecca Nurse. Indeed, the only way that goodness can be proved, arguably, is through the way in which they stand up to or fail to stand up to the test that each of them face and pass. Each of these characters, finally, meet their deaths knowing who they are more than other characters. This is most clearly shown by John Proctor and his waivering, deciding to die to save his "name," because he realises at the end of the day that who he is is actually the most important issue.
Much of what is going to be present in the introduction is going to come out of what your analysis of the characters are going to be. I think that it would be a good starting point to use the title of Miller's work in your introduction. The very idea of discussing a "crucible" could be important in the overall thesis of the paper:
A crucible is a container used to heat metal to extremely high temperatures, refining it to its barest essence and melting away any foreign substances or impurities. A crucible is also defined as a 'severe test or trial, especially one that causes a lasting change or influence.'
Given how the events of Salem will impact the townspeople on both personal and social levels, I think that it might be effective in the introduction to stress how the entire play surrounds this idea of a "severe test." The reaction of the townspeople to the accusations of witchcraft, and how social solidarity endures its own crucible as a result would be effective elements in to include in the introduction. At the same time, I think that bringing out, briefly in the introduction, how Rebecca, Giles, and John all must reckon with the consequences of this test, recognizing how their characters are only worthwhile when put through these intense crucibles. I think that the introduction of your paper including the actual definition and term of "crucible" might help to establish what is going to be proven in the course of its analysis.