Oh, I like all of those headcanons, especially the grocery store one. Very Sherlock.
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The biggest challenge for Sebastian is that, while he understands humans well on a psychological level despite his not being capable of feeling all the emotions humans do, he's kind of ??? about many of the finer details on how their physical well-being works. He knows plenty about the most efficient ways to kill a person, but apparently this is the first time a contract has required him to take care of humans, and he's never interacted much with children before. He's been serving Ciel for three years, so by now he's learned that, okay, you can't pour boiling water straight into the bathtub because that's too hot, and apparently there's such a thing as too sweet even though humans like sugar? Go figure.
Still, he might slip up on details, even simple things any human being would know. Say, if he were suddenly supposed to account for what having a cold feels like, what the effects of sleep deprivations are, or how long a papercut takes to heal? "...Uhh /bs something and hopes it's accurate".
He also has a bit of a problem toning down his awesomeness to appropriately human levels, partly because it's just so natural for him and partly because he wants to do everything perfectly. He's better about it than he was when he entered his current contract, but Ciel still has to kick him about being too much of a show-off sometimes. He tends to forget about basic human things at times, too - he'd easily forget he's supposed to eat if it weren't for people around him eating and reminding him that way, and there's this one silly side-chapter where he's offered alcohol, downs about twenty shots in the span of a few minutes and doesn't even think to feign drunkenness. He's a complete loser, honestly.
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Do you have any particular strategy for Sherlock's ability to read people? How do you deal with canonblindness in regards to that?