What has to happen? This is the prompt I use most often. It is not immediately obvious why this is such a great prompt. Let me explain.
At the beginning of your story, there will be a situation – either good or bad opportunity. So you just ask the question ‘What has to happen for my character to start the quest?’
Ask the question at each step of the character’s journey. Ask ‘What has to happen for my character to doubt the quest?’ and you will find the answers spring to your head.
And listen to your characters. They won’t react the way you expect them to react. When our boiler had a leak I called the plumber, but I know a couple of guys who would try to repair the boiler themselves (usually with very little success and causing untold grief and disappointment – it made a great story).
Now that is what had to happen!