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I did this once in a novel-length story. After the first two appearences my wife said that she has a strange feeling with the side character, but couldn't tell why. In the end she was like 'I knew ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47834 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
I did this once in a novel-length story. After the first two appearences my wife said that she has a strange feeling with the side character, but couldn't tell why. In the end she was like 'I knew it!'. What did I do? I made the side character appear 'just in time', when noone else was around or 'hide' from others for whatever reason. She did always know what was the right thing to say, what the protagonist needed the most in that exact moment, being 'the one she adores'. So you can make the audience question the existence if nobody else ever talks about this character despite she is somehow always there. You can make her a bit too insightful, give her a bit too much influence on the protagonist, in short: make her feel **slightly** off without breaking the suspension of disbelief. Just make her totally normal for the protagonist.