...By trying to catch it. If it flies outside the map, leave the room and enter it again.
I'm an honest man
There are only two characters who change their expression, and that is via a special stone found behind the demon king's throne.
Also, if you want an NPC game where their expression change, you could try this:
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There are quite a bit of NPCs, and their expression change during sex and their clothes change after having sex a few times.
It has some loli stuff, so you might want to watch out for that.
I'm actually planning to upload that. I've already MTL translated it, but I'm trying to make it sound more intelligible since, well, this MTL genuinely made me feel as if I have a committed murder.
Hope it doesn't get taken down, though.