How am I supposed to improve my ability to think of quotes on the spot when doing timed exams?

In-depth character and theme revision will help you to memorise some key quotes, and over time, more universal/overarching quotes will start to become more prevalent in your revision, and these should become favourite quotes that are more likely to be useful in exam questions. Also, when revising context, wider context and (when relevant) critical views, make sure to make this revision essay specific, applying the revision to quotes/points you would make in your essay, as this will help you memorise both the context and critics, as well as the relevant quotes.