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It all depends on your intention. 1) You want your reader to be suspicious about the character, but not know the true explanation. In this case you can put spotlight on this character, show some m...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/27116 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
It all depends on your intention. 1) You want your reader to be suspicious about the character, but not know the true explanation. In this case you can put spotlight on this character, show some mystery and hidden depth, but leave a number of other possible explanations to his actions. 2) You want your reader to be totally oblivious, and only in last chapter smack his head - "How could I miss that?!" In this case you have to keep the spotlight somewhere else. The character should be fully consistent with his assumed identity, and if there's something strange going on, there always must be some other suspect. In both cases you have to introduce the idea of a persona who is the real identity of your character. Shroud this persona in mystery, make the other characters look for him and try to learn about him without knowing that he is hiding in plain sight. P.S. I think J. K. Rowling is using the idea of false identity too much, almost to the point that I know that newly introduced major characters are rarely the people they are pretending to be.