I suppose a lot of stories could start with an event that took place in the area where you live now, many years ago. A murder, a bank heist, a disaster, a kidnapping, someone who took on a large corporation or the government and won etc. After choosing one of these, you need a hook, something to make it different from all the other types of similar movies that have been done before. Take the Blair Witch Project, near the end when there is just the man and woman remaining, and they are walking through the woods, you think they will find the road, locate their car or stop someone and get help...but oh no, the big scary house appears out of nowhere and we all know what happens to them...

If you have older relatives living nearby or friends who've 'been there, done that and bought the t-shirt', they can also be a good source. Newspapers, the internet (everything you read on the internet is true, isn't it?), the movies - better to give a genre a new twist of your own rather than trying to rehash something that has been done before. So research and brainstorming would feature high on my list of priorities in that initial stage to get a good solid idea that would stand up to all kinds of scrutiny.

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Better than a poke in the eye with a pointy stick any day...