Let's just great straight to the point here, shall we? Plot twists can only and may only happen when a character is depressed, it's a mystery novel, or your book is moving at an intensely slow pace. These might happen all at once, but very (very) rarely does that happen to the writers we have today. Unless you're a certain vampire that sparkle and werewolves that have nothing to do with the moon fan *Cough, cough* Twilight. *Cough, cough*.
A depressed character could start off depressed, or become depressed throughout the story, it doesn't matter; they are depressed in any form. A plot twist can cause the depression, happen while the depression, or at the depression. An example of at the end, would be the depressed person's suicide, which is a Cause and Effect. The Cause: suicide. The Effect: The death which causes the end of one's life and depression.
A mystery book has to have plot twists, or it can't be a good book for the reader. Imagine if your sitting there, starting to read a mystery novel, wanting some murder and gore, but it takes most of the book to get the plot twist. How boring! YOU MUST HAVE A LOT OF PLOT TWISTS DURING A MYSTERY BOOK!!!! You won't have a good book if you don't.
An intensely slow paced book is never good. When you write a book, you want to keep the readers on their toes and leave them wanting more of what they're reading. A super fast-paced novel is never great either, unless it's a children's book; that's the only place where it can be okay. But when your writing or ages ten and over, you need it to be a good pace of having some places drag and some places being as fast as lightning. A plot twist in here can cause the reader to be on the edge of their seat and want to read more of your book. An example: Somebody finds something that they lost a very long time ago, or a long lost relative comes to visit. You at least want three to six plot twists for the average book.
this is where i ramble; have fun