What worked for me was just bullet points of main ideas (you should make sure you have something substantial to say on all the characters and all the themes that come up), in some order, to stop it getting crazy disorganised, then fleshing things out when I went along. Organise everything in terms of point, evidence (quotes, learn lots of them), explain (what is the author's intention). Also might be useful to organise things by assessment objectives, so maybe assigning things like specific historical context to each of your main points (depends on exam board and what is being assessed in the particular question)
what to include depends on the question. If it is on a character, you can talk about how that character is relevant to the themes and what point Priestley is trying to make with them, if it is on a theme, you can talk about how different characters/sections display that theme and the point behind the theme..