Some claim Alfred Hitchcock coined the word. He defined it as the object around which the plot revolves but which the audience doesn't really care about. George Lucas disagreed and said, "the audience should care about it almost as much as the dueling heroes and villains on-screen." Another filmmaker, Yves Lavandier claimed it was what motivates the villain even more than the hero. So, in the Maltese Falcon, the MacGuffin is what drives the characters' actions and yet ends up insignificant.
Does every story need one, of which the writer consciously knows-- or will there be one whether the writer knows or not--
When I first heard about MacGuffins, I had already written quite a few books-- without thinking of whether I used one. As I looked back at each plot, I saw they had one-- or was that more than one? It happened in the writing-- because I needed something that would make the actions of the characters be logical.
MacGuffins became of renewed interest to me in the series I am currently working on since it is romance, suspense, and paranormal. Ordinary people are thrust into adventures, but the reason for what they do has to be believable to the reader. The stories, with no cliffhangers and each romance complete, must hang together. A carefully considered MacGuffin is what I saw as the answer, and it led me, as I laid out their basic plots, to find the MacGuffin in each.
Romances didn't show up in any of the articles I read on MacGuffins, but I see them as prime examples of how it works to add depth to a story. The MacGuffin for a romance won't be finding a one true love-- because it's at the heart of that genre. So the MacGuffin has to be the 'other' thing that drives the hero and heroine, but in the end, didn't matter as much as they thought. It keeps the action going long enough for them to realize what they truly want.
What I am considering now if there might be one MacGuffin for all five books...