Although Goldman's The Princess Bride is as much parody of fairy tale as fairy tale, there are any number of fairy tale elements in it. If you are not familiar with the elements of a fairy tale, some of the most common are: signal phrases for beginnings and endings, such as "once upon a time" and "happily ever after"; magic words or phrases, like "abracadabra" or "open sesame!"; sharp distinctions between good characters and evil characters; disconnect with the real or historical world, with stories taking place in an "enchanted forest" or faraway kingdom; magic; false death or sleeping sickness; happy endings; moral messages; royalty and castles; non-human creatures; and magic items.
Choosing just one for The Princess Bride, I would suggest repeated "magic" words and phrases. Three clear examples are "true love," "as you wish," and "My name is Inigo Montoya..." They might not seem like traditional magic words but each does have a magical function within the story.
From the beginning, the reader or viewer is taught that "true love" has a special kind of magic. It can overcome all obstacles, and even death cannot stop it. It is considered a cause worthy to sway even the faintest-hearted heroes to action. It is so powerful that Miracle Max, who hoped to avoid having to perform a miracle, is terrified when The Man in Black tells him true love is what he has to live for. He tries to lie about what he heard, because if it really is true love, then Max cannot (or at least should not, if he is going to be a good character) fail to act. Another instance of its power may be seen when Wesley tells Buttercup that even death cannot stop true love. Here, the phrase has descriptive power, rather than the power of magical utterance. By Wesley's reference to true love, the reader or viewer is reminded that it was his response to Miracle Max that made his return to Buttercup possible.
"As you wish" has a different magical power. Rather than overcoming obstacles, the phrase removes the disguise that keeps Wesley hidden from Buttercup. Unlike the magic words of most fairy tales, the reader learns how this phrase came to have its power. They learn of the first time it was used and how Buttercup knew when the stable boy said it, he meant that he loved her. Instead of functioning by any supernatural magic, the phrase achieves its power by association and memory. In this way, it also serves as critical commentary on "magic words" in fairy tales, suggesting their link to history, memory, and identity. Only the hero with the right words can pass the test.
Finally, Inigo's repeated use of "My name is Inigo Montoya. You killed my father. Prepare to die," functions magically by combining the powers of the first two phrases. Inigo has been telling himself that he will say this to his father's murderer ever since the day that his father was killed. It serves both as an origin story and a vow. Origin stories or histories have the magic of identity. They can serve as touchstones for the speaker or revelations for the listener, in the same way as Wesley's "as you wish." A vow can also be defining, but it has the additional force of passion and will invested in it. Like "true love," a vow of this kind has the power to overcome death. When the count nearly bests Inigo and he feels himself dying from his wounds, he begins to speak his magic words over and over. The words give him the strength to continue the fight and defeat the count.
In the fairy tales that lie behind The Princess Bride, magic words have true, unexplained magical power. In The Princess Bride, their power is revealed and explained, and yet, in the end, there is still that hint of magic about them in the way they enable their speakers and hearers to do extraordinary things: defeat death, see through a mask, summon heroic strength. Although the power might be that of mind and memory, the reader or viewer is never allowed to be one hundred percent certain that there is no supernatural magic involved.