How do I plan an essay under timed conditions and generate ideas quickly?

The first thing you want to do is really make sure you understand the question: this is something that is worth spending time on. Questions are often provocative for a reason: think about what tensions come from what you're being asked - do you instinctively feel strongly either way? If the question is statement, like a quotation, try and rewrite it as a question. For example 'Explore the role of Enobarbus in Antony and Cleopatra' could become 'What would Antony and Cleopatra be like with Enobarbus?'

This can become a productive method of forming an argument - spend 1-2 minutes jotting down all the questions that come into your head when thinking about the essay question. Some of these will naturally link together and follow on from each other, and this can help form the structure of the essay, as well as give it a natural feeling of moving forward. You can then write quotations/key moments next to these questions, and once you have a long list pick out what you think is most interesting. Write out your argument in one concise sentence at the top of some scrap paper, and make sure you come back to this at the end of every paragraph.