NOTE: This article appeared twice - first as "Twist Endings" and then as a reprint in a non-paying market as "A Touch Of Twist Endings".
The twist or surprise ending, also called a "switch-over" by H.R.F. Keating, requires extensive planning to succeed without leaving questions in the reader's mind. There are four primary twist ending types (with assorted variations).
In one you come up with two good endings that can be deduced with the clues available, but have one small clue that points to one solution over the other, which while given is passed over as an insignificant background detail until the end (a la Rex Stout).
In this instance the plotting requires that 1) the multiple endings be slowly revealed in the characters' background. 2) The surface motivations for the character's actions hide the true goals. And 3) that the clues have double interpretations.
A variation on this method uses the second element where-in a character sets out to achieve a purpose which is made clear to both the reader and the opposition. This false goal requires such extreme measures on the opposition's part that the true goal is thereby achieved.
In the second twist ending division the reader sees the action through a character's mind. This character has strong convictions and perhaps logic tight compartments. The reader might be aware that the world view is restrictive - depending on how like the character s/he is - but may not take that into account until the end when the character is shown to be totally wrong.
Alfred Hitchcock has been known to use this method in his films. One that comes immediately to mind is North By Northwest where-in Cary Grant's character believes that the female lead is a villain, when in fact she is an undercover operative.
Three variations on this method have been used by Agatha Christi and Richard Hull. In Christi's book The Murder Of Roger Ackroyd the story is told from the doctor's viewpoint playing "Watson" to Hercule Poriot's "Holmes". The surprise comes when we discover that Doctor James Sheppard is the killer. Through-out the story Christi faithfully gave the clues, but so subtly they are easily passed over by the reader, until the end when Poriot solves the puzzle.
Christi used a second variation in her Witness For The Prosecution, wherein the wife's treachery is shown in the end to be self-sacrifice to prevent justice being served on her guilty husband.
Richard Hull used semantics as the base for his twist ending in The Murder Of My Aunt. The story is told in a journal written by the nephew in the story. He is a nasty, parasitic, young man who plans to murder his aunt for her fortune.
The twist comes when the reader discovers that the aunt found and read the journal, and being forewarned kills the nephew. The title of the character's journal and the book The Murder Of My Aunt being there-in shown to have two possible definitions.
In the third main method the operating element is Murphy's Law. Everything is going according to plan. Then believably and reasonably within the story's context some small (foreshadowed) element that the characters overlooked becomes a major stumbling block. This throws everything into a kilter. Donald Westlake uses this method in all his Dortmunder books, not as an ending but to heat up the action - usually in midpoint.
The fourth method for developing a twist ending is much like building a stand-up comic's jokes, except that there is a lot more story before the punch line. The "gag" story relies on the quick laugh. The action is fast paced. There is no time for detailed characterization, yet characterization must be there within the action. How your character does an act tells more about him than the act itself.
Studying Bill Cosby's stand-up humor is your best guide in developing your own versions. Gag stories range between 300 and 1,000 words, with the best between 400 and 500 words.
While I used primarily mystery stories as examples, the twist ending is viable in any genre. For a helpful twist ends list I suggest you refer to pages 52 through 55 of Martin Roth's book The Fiction Writer's Silent Partner.
Whatever twist ending method you develop the reader must be given a clue to the true situation. The language in which the clue is given is important. The idea is for the reader to assume one thing when in reality it is another. The reader is encouraged to misinterpret the information.
A simple example would be a love story that seems standard until it is revealed that the lover is the same gender, a ghost, or a fantasy in a psychiatric patient's mind.
Whatever the method, the foreshadowing must be done with a delicate hand for the ending to be a surprise. The writer must make the ending satisfying as well as surprising, so that the reader can go back and say, "Oh, yeah! Of course!"