One thought: a prime example of the "things being just slightly off" thing is when people start to be animal-like, or animals like people, in a way that's not just understandable behavior, but sort of viscerally disturbing. (Like, that's part of the creepiness of the stalking in my nightmares, I think -- people start staring at me like they're predators, large cats or something.) With your characters changing back and forth occasionally, that could play into things a bit.
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One thought: a prime example of the "things being just slightly off" thing is when people start to be animal-like, or animals like people, in a way that's not just understandable behavior, but sort of viscerally disturbing. (Like, that's part of the creepiness of the stalking in my nightmares, I think -- people start staring at me like they're predators, large cats or something.) With your characters changing back and forth occasionally, that could play into things a bit.