One thing that characterizes mystery novels more than, perhaps, any other genre is the plot twist. Plot twists keep the reader interested and guessing. They also add to the richness of the story. For instance, in Agatha Christie's A Holiday for Murder (AKA Hercule Poirot's Christmas), two of the important characters turn out not to be who we think they are. That fact isn't central to the plot, but it's interesting, and it keeps the reader engaged.
Plot twists also add to the suspense, and suspense is essential to a good mystery. For example, I like Steven White's use of suspense in Manner of Death and his other Alan Gregory novels. Perhaps the master of plot twists and suspense was Alfred Hitchock. He wasn't a novelist, but he was a genius at the unexpected, and was better able than almost anyone else to create real suspense without resorting to graphic violence. That takes real talent.
Some plot twists get the reader's attention right away. For instance, in Joan Smith's A Masculine Ending (the first in her Loretta Lawson series), Lawson wakes up in a Paris apartment she's borrowed to attend a conference only to find someone else sleeping in the next room. When she comes back later, she finds the person missing, and only a bloody bedsheet to show that anyone else was there. That plot twist happens very early in the novel, and Smith keeps the reader intrigued throughout.
That's one of our main goals as mystery authors. We want our readers to stay interested the whole way through the novel. We want the reader to want to find out what comes next. One of the greatest compliments I've ever gotten was from a reader who told me she stayed up late reading the night she got my book, Publish or Perish, because she wanted to find out what happened. We mystery authors love that!
The fact is, though, that creating plot twists isn't always easy. A good plot twist has to be reasonably believable, or it doesn't keep the reader's interest. It also has to come at the right time in the novel, so that the pace of the novel is just right - not too fast or slow. Too many plot twists can be confusing, but too few can be boring. A plot twist should be enough of a twist to keep the reader involved, but not so much of a twist that the story becomes completley improbable. There's a lot to consider when it comes to plot twists, so as I'm reviewing B-Very Flat before I send it to the publisher, I'm paying attention to the twists and turns of the plot. I'm trying to make sure they make the story better, and not more confusing. That's not as easy as it sounds ; ).