You are required to find critical interpretations to include in your research paper.  In critical interpretations, scholars use theory to study, analyze, evaluate, or interpret a play, characters in a play, or themes in a play.  In addition to finding books about your play or playwright (using PittCat), you should find some scholarly articles.  Scholarly articles are written by an expert in the field and are often reviewed by other experts in the same field prior to publication.  This means that they have a higher level of quality than magazine or newspaper articles.  Most databases give you the option to limit the results list to include only scholarly articles, and others, like JSTOR and Project MUSE contain only scholarly articles. 

Here are a few databases you can use to find scholarly articles...simply type in the name of your play or playwright to get started!

This is a great, short video from RMIT University in Australia that explains what scholarly/peer-reviewed/articles and library databases are.