Virtually all dramatic, emotional, thrilling, or satisfying moments in films center on the protagonist's choosing one course of action over another. In order to deliver these moments effectively, the screenwriter needs to keep the protagonist's decision-making process as clear and understandable as possible -- because if the audience doesn't understand what his or her options are, or if there are simply too many options, then the drama is lost.
Theoretically, we all have an infinite number of options in just about any situation. If a waiter asks if you want coffee, you can also respond with any number of non sequiturs ("White elephants on Tuesday!" for example), or run screaming from the restaurant, or throw your chair across the room, etc., etc. However, most people would consider the only two "real" choices to be "Yes, please" or "No, thank you," and if a scene like this appears in a movie and the protagonist says "Yes, please," we don't need to explain to the audience why he didn't elect to throw his chair or run away.
But there are plenty of other situations where the number of "real" choices is much, much larger. If you're a single person in Los Angeles, how do you decide whom to date? How do you even decide how to look for a date, given the plethora of both real-world and online mate-finding resources? The fact that you can actively search for a decade without exhausting these options is more likely a source of frustration than relief -- and that's true whether we're talking about this scenario as a real-life dilemma or the plot for a romantic comedy.
Let's say we're trying to make a go of the latter (we'll call it Untitled Los Angeles Dating Comedy... or, perhaps, to get some heat behind it, Untitled Rachel McAdams Project). At the start of the movie, Rachel stands at the edge of the nigh-limitless dating pool that is L.A., in the exact situation we've just described. By the end, we assume, she will be paired up with some charming, eligible man. In between, there will be shenanigans, misunderstandings, and soul-searching. Throw in Amanda Seyfried as the acerbic best friend (Mean Girls reunion!), open it against a big action movie, you've got a hit. Easy, right?
Sure. But there's a catch: Even though Rachel will technically be choosing her ideal guy from among an entire city of bachelors, we don't want it to look that way in the movie. Because if we really present it that like that -- as if any of this sea of men were a potential mate, and she just happened to choose this one -- then we've got an exceptionally weak ending on our hands. The audience leaves wondering, "Why him? Why not guy #10273, or the one fifteen down from him, or anyone else?" And we haven't done our job as screenwriters.
This is where the concept of funneling comes in. As the story progresses, we must continually find ways to narrow Rachel's options -- not widen them. And we have to do this in a believable, transparent way. Like this:
Scene 1: Rachel tries speed-dating. The event is full of guys from the investment banking world, and she hears one boring financial story after another. By the end of the night she knows damn well she's never marrying a banker.
Scene 2: Rachel goes with a friend on an Art Walk in Venice. She gets hit on by all manner of starving artists, but can't believe the level of entitlement and smarminess on display, considering most of them couldn't afford to buy her coffee.
Scene 3: Rachel goes on a blind date that her cousin arranged for her. He's a major douche and it's an unmitigated disaster.
Now we're making progress, because we've reduced her choices. The audience isn't going to be left with the nagging feeling that Rachel should really go back and give those i-bankers another shot, or ask her cousin to fix her up with someone else. However, as the dating pool shrinks, the viewers are going to be wondering just what kind of man is finally going to do it for her. In other words, they want to know what happens next -- and that's what we want them to want.
Things don't necessarily work this way in real life. If the above three scenarios actually happened, it's unlikely that each of them would be so clearly negative. Chances are better that they'd all have their high points and low points, and that afterwards, if Rachel were honest with herself, she'd say that there were legitimate reasons to keep all three options open in the future. But if we wrote the movie that way, it would be an incredible bore. No progress, no momentum, nobody awake by the time the credits roll. To keep the audience's attention, we have to keep forcing Rachel down a narrower and narrower path. Thus, a million guys becomes a thousand; a thousand becomes a dozen; a dozen becomes one.
But wait! There's a catch. Once that dozen has become one -- once the Best Guy in L.A. has been revealed -- we are absolutely within our rights to turn the entire funneling principle on its head and introduce another seemingly perfect guy (someone from Rachel's past, someone who's previously rejected her, someone she rejected who's changed, etc.). In this case, widening Rachel's options works to create drama rather than kill it -- and this is really the only time we can get away with doing so.