Yes, like JSBACH, it's almost exclusively spoken language for me, I don't tend to pick up body language or other habits. The way that I do this with accents and dialects has almost got me into trouble a few times, when people have thought that I'm mocking the way that they speak - but I don't do it on purpose, and often don't notice unless it's pointed out to me. If I realise myself that I'm doing it, I have no idea how long I've been doing it for. Oddly, if I'm asked to deliberately talk in a different accent, my impersonation is usually terrible!
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When you are fighting an invisible monster, first throw a bucket of paint over it.