3) Keep your mysteries hidden, like Pettigrew:
This point is similar to the one above, just not limited to backstory. What happened to your interest after you discovered who shot J.R. Ewing (if you're old enough) or Mr. Burns (if you're not!)? And where did your interest go after Nanny Fine married Mr. Sheffield? Once a mystery is solved or question answered, the viewer, or reader, quickly loses interest. Wanting to know a secret, to solve a mystery, to answer a question is what keeps the reader glued to the page.
Even if your story is not a genre mystery, it still must contain a lure of some sort to keep the reader hanging on. Plot these threads and the release of information well so that the reader must...keep...reading…until the very last Elder Wand owner is revealed!
Over the course of seven books, JK Rowling plotted a complex mystery embedded in the frame of a fantasy adventure. Ms. Rowling expected a lot from her readers, and she got it. She expected an active participant to pick up on her clues and to follow their trail. What she got was a world full of Harry Potter readers who not only jumped in enthusiastically to sleuth out the clues, but also delighted in stringing them together to plaster the Internet with theories of what was yet to come.
There are three central questions to the Harry Potter mania which drove the search for clues:
1) What exactly happened in Godric’s Hollow?
2) Where did Snape’s loyalty lay? and
3) How would Harry defeat Voldemort?
Although the three questions are simple, the answers are quite complex. JKR built an elaborate world, richly detailed, that is full and complete. What is amazing about her construction is that every aspect of the world, each character, has something to contribute to these three simple mysteries. The clues could be, and indeed are, hidden everywhere.
While JKR used multiple techniques for hiding her clues, overall they can be categorized under that old reliable magician’s trick: sleight of hand…the art of distraction.
Misdirection is perhaps the most important component of the art of sleight of hand. Using misdirection, the skillful magician choreographs every movement in a routine so even the most critical and observant spectators are compelled to look where the magician wants them to. (source)
JK Rowling is the indeed the master magician! While planting her most important clues, she diverts the readers’ attention away from it and to her carefully plotted distraction. Aside from a few obvious hints meant to knock the reader on the head to hunt for more, Jo almost never plants a clue without also providing the words to distract the reader from it.
In Chamber of Secrets, in Chapter 9, The Writing on the Wall, the Trio enter Myrtle's bathroom for the first time to hunt for clues as to what petrified Mrs. Norris. Once they leave, Percy confronts them, chiding them for returning to the scene of the crime.
"Why shouldn't we be here?" said Ron hotly, stopping short and glaring at Percy. "Listen, we never laid a finger on that cat!"
"That's what I told Ginny," said Percy fiercely, "but she still seems to think you're going to be expelled; I've never seen her so upset, crying her eyes out. You might think of her, all the first years are thoroughly over-excited by this business--"
"You don't care about Ginny," said Ron, whose ears were reddening now. "You're just worried I'm going to mess up your chances of being Head Boy."
"Five points from Gryffindor!" Percey said tersely, fingering his prefect badge. "And I hope it teaches you a lesson! No more detective work, or I'll write to Mum!"
I imagine Jo chortling to herself as she wrote that sentence about "no more detective work." The italics there are hers, not mine. Of course she's wanting her reader to do exactly what she's having Percy forbid the Trio from!
As for the sleight-of-hand in this example, I see two types. First, the clue is Ginny's extreme reaction to what happened to Mrs. Norris. However, Rowling distracts the reader from Ginny's emotional reaction with a fight between Ron and Percy. She also uses a character's own false perceptions to distract us from the real reason why Ginny may be upset. According to Percy, who is a Prefect and talked with Ginny so should be trusted to know, his sister is upset because all first years are and she's afraid Ron will get expelled.
As a writer, you must play fair with your reader. You must leave clues. But the sly author will do it in such a way so as to make it difficult for the reader to see them clearly on a first read.
Plotting a mystery is a very fine balancing act. If the author leaves insufficient clues to give the reader a shot at solving the puzzle, the reader feels cheated. However if the author makes the clues too obvious, the reader also feels cheated out of the pleasant surprised “gotcha” at the end.
JK Rowling masterfully employed sleight-of-hand to give her readers this satisfaction.