In Ch 20, Too-clever Henry has a little jest with Catherine's supple imagination as he describes the horrors of the classic Gothic Abbey on their way to Northanger.

Catherine is already in awe of the General's fearsome punctuality, intimidation of his children, and impatient temper when things don't go exactly as he wishes.

So, harness up a rather (1) "Gothic" character in the General with (2) Henry's playful but overly influential description of the abbey and (3) finally throw in Catherine's horrid novels and you have a recipe for turning Catherine's fanciful thoughts into something like a not-so-imaginary paranoia!