The Grownup was originally part of a short story anthology put together by George R.R. Martin and it is very typical of Gillian Flynn’s style. Basically it’s a mini version of the role reversal plot that was used in Gone Girl. A fake fortune teller is asked to purify a household but then finds out that the character who is supposedly possessed and the woman who hired are not exactly as they seem.
It’s a quick story that’s a bit creepy and clever. Nothing else to say about it. Do seek it out. It’s good but not WOW.