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Very simple: Write the story up to the point where someone is supposed to die. Then roll a die, and kill off the person whose number comes up. You don't literally have to do that, but if you want ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/36003 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Very simple: Write the story up to the point where someone is supposed to die. Then roll a die, and kill off the person whose number comes up. You don't literally have to do that, but if you want your character deaths to be unexpected a character they to essentially be a random events from the readers perspective. It makes it somewhat easier to actually pull this off from a writers perspective to actually do it in a fashion similar to that though. Note that this will be _very_ unsatisfying to many readers. You should also note that this doesn't actually matter, unless your target audience wants a more predictable story. I personally prefer more structured stories where deaths are meaningful if they happen at all and there's a happy ending awaiting those who finish the book; But that's not the kind of book that you're writing if you genuinely want your deaths to be unpredictable, and you obviously don't need to write your book for people like me. If on the other hand you want deaths to be meaningful and have an impact beyond "I didn't want that character to die because I _liked_ that character", your deaths _won't_ be unpredictable. If the people die in a non-random fashion you'll set this up somehow, and if the audience doesn't catch on to your setup it just means you did it poorly.