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I think that one of the important things about a character like this is that there has to be some kind of consistency. You can have the wild mood-swings depending on who he is interacting with, but...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/25434 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
I think that one of the important things about a character like this is that there has to be some kind of consistency. You can have the wild mood-swings depending on who he is interacting with, but I think there needs to be some kind of base-line, something which tells (or even just hints) that this character has some standard of behavior. It's an interesting idea, but it begs the question - "What happens when he is not around other people?" If I understand the character correctly, he consistently imitates anyone he is interacting with. The problem that occurs with this is that when he's NOT around other people, he has to behave in some way, even if it's sitting in a chair, staring at the wall. Even if the audience never sees him by himself, or doesn't see much of him at all, you as the author should have a good idea of how he behaves in many different situations. In this way, you have a greater understanding of him, and this will inform you about why he behaves the way he does.