I was just thinking about the interesting and counterintuitive use of MacGuffins in film noir.
A little background. Hitchcock—and most suspense writers—use MacGuffins to retard the love story. A MacGuffin pulls a narrative down a track criss-crossing the love story that will make the movie end happily. We want the happy ending, but not too soon—we want (as George Ainslie says in his great essay on "Money as MacGuffin") to build up an appetite for it not ruined by being satisfied too soon. That's why we go to the movies instead of just day-dreaming all the time: we can have anything we want in our day-dreaming restaurant, and so we never end up really wanting anything. But movies—even romantic comedies—delay the happy ending and make it all the more rewarding by doing so. Still, as Ainslie says, it's not just a question of waiting for the happy ending: it's seeing it blocked, and often more and more blocked as the tell-tale compression of minutes that remain (to allude to Northanger Abbey) seems to make the happy ending less and less likely, more and more a mere daydream. That's how climaxes work: there's no way out! Ginger Rogers has actually married Bedini. Almost as bad as when Ingrid Bergman actually marries Alex Sebastian. (I am using stars' names and character names advisedly: part of the point is the star we want to see in a happy ending in this movie marries: some other character, and not the other star.)
So because we don't control the fiction, we worry that we won't get the reward the desire (or "literary need," as I've been calling it in earlier posts) for which has been building and building. Okay—that's one way of describing the most basic plot: want something and wait for it. But what's great about Hitchcock, and Hitchcockian plots in general, is the way he counterpoints that desire with another one: the desire to know the significance of the MacGuffin.
It's not that the MacGuffin is just an objective the hunt for which brings the lovers together. That's a pretty standard plot too, and it makes sense and it works. But in Hitchcock, the MacGuffin is always a puzzle. "What are the 39 Steps?" Why does the Lady Vanish—what could her significance possibly be? What is it that James Mason is trying to sneak out of the country? What's in the wine bottles that makes them so significant? Of course we sometimes know what the objective (really) is in Hitchcock, but his movies seem deepest when we don't.
So this makes possible the interplay of two stories: one about love, the other about knowledge. (Vertigo telescopes them together in interesting ways.) The love story can end at any time if the lovers just walk away, which is just what the male characters keep urging: Cary Grant to Ingrid Bergman, and to Eva Marie Saint, Robert Donat to Madeleine Carroll. (Occasionally, and interestingly, it will go the other way, especially if the male star is Jimmy Stewart, doggedly ignoring the good advice of Kim Novak or Grace Kelly: but then she gets into it.) So we could get the reward of the love story with a happy ending if we gave up on the knowledge.
But we also want to know, so that we find ourselves torn between two conflicting preferences. Go away or search the wine cellar? Figure out what James Mason is up to, or check in to a lodge at Mt. Rushmore? And the audience always chooses knowledge, deferring and risking the love story in order to try to have it all. It's important that the knowledge is never worth it—it's a gap more interesting than its solution. (How couldn't it be? Narratives about the search are always more interesting in their middles than in their endings. Mistah Kurtz, he boring (even when played by Marlon Brando); Alaska, she dead (spoiler, sorry!).
So the end of a Hitchcock movie gives you a quick—a very quick—revelation of what the MacGuffin really was. Our thirst for knowledge is slaked, and because that slaking can't be of the order of the desire it satisfies, the solution would be (often is) a disappointment if there weren't something else converging with that conclusion: the consummation of the love story. So the MacGuffin is the mechanical white rabbit (or undetached rabbit's foot, as in the Quinean Mission Impossible III) that leads us down its own track or rabbit hole to the deferred and longed-for conclusion.
So back to noir, and the femme fatale. The genius of movies (and sometimes novels) like The Maltese Falcon and Out of the Past is that the MacGuffin's formal status as MacGuffin is part of the plot, that is part of what the noir (anti)hero is doing. Spade or Jeff Bailey are not, or not fundamentally, in love with the femmes fatales (disclaimer: yeah sorry, the movies are sexist, it's the formal structure I'm interested in). They both know, and know from the start of the main action, not to trust the women they are teamed up with. And they're barely interested in the MacGuffins, the "dingus" as Spade calls it, at all. Rather they want to understand the crimes that have organized themselves around the MacGuffins. It's still a question of knowledge. Spade knows who done it from the start (as we find out at the end), but not why. So the forties noirs use what might be called fake MacGuffins, objects the detectives are not really interested in, even in the fictional world, and fake love stories, stories for which the detectives have no ambition or dersire for a happy ending, in order to find out the MacGuffin of all MacGuffins, the truth.
That means, of course, that noirs are about truth rather than erotic satisfaction. But the truth is about human character, not the value or history of the dingus (even if it's worth as much as Dr. Evil's "ONE MILLION DOLLARS!"). The noir detective uses the MacGuffin to find out the truth about human motivation. That truth is not erotic, and so the MacGuffin in noir doesn't lead us circuitously to erotic satisfaction: the detective uses the MacGuffin as a decoy that seems to map out that circuitous route, in order instead to undermine the erotic interest in favor of the truth. In Hitchcock the search for truth is a well-paced route to love. In noir the detective controls the pacing of the falsified love story through the MacGuffin in order to find the route to the truth.