Spend some time getting to know your topic. Identify any synonyms, the language used in overview/background articles, related concepts, and people. See the mind map example on "Game of Thrones"
Find your path as you explore. What are the keywords along that path? Perhaps you find information that leads you to a statement you think you can support:
"The costumes in the Game of Thrones contain clues to evolving character development."
What are the keywords in this statement to search for articles written by scholars or authorities within the subject area?
costumes AND Game of Thrones AND character development