According to Wikipedia, the stock phrase "Once upon a time..." has been in use in some form since at least the 14th century. And its prevelance is not just limited to the English language - the Wikipedia page lists variants in dozens of languages from around the world - as also the modern variants, "A long time ago..." and even "Not so long ago..."
While all of us have heard at least one story that began "Once upon a time...", we are also acutely aware that some lines make for great story openers - if only because they unambiguously announce the intention to narrate a story.
(The line that did the trick for me during my childhood was "For those who came in late..." enshrined in the opening panel of every Phantom comic book, as seen above.)
But is there something else at work here? Do great (or stock) opening lines do more than just build the expectation of a narrative?
I would like to think so. At least after reading up research findings that the mere act of passing through a doorway clears up your memory and begins a new memory episode (Via). Making it less likely that you'll remember something that happened in the room you just left. The unlikely result of simply walking through a doorway to another room is akin to wiping your memory slate clean.
The effect itself is not dependent on the distance walked but only on the act of walking through a doorway. As the reserachers put it "Walking through doorways serves as an event boundary, thereby initiating the updating of one's event model [i.e. the creation of a new episode in memory]."
The findings also complement so-called episode markers in story-telling - phrases like "a while later" seem to create a temporal boundary within the narrative. As a result, test subjects found it difficult to remember the sequence of sentences in an episode prior to the narrative divide vis-a-vis sentences in the current episode.
So, here's what I think is happening with the stock opening phrases or with great opening lines.
An effective opening line creates a temporal boundary between what you were doing or thinking before the story began and afterwards. Or in other words, the opening line is transporting you across an imaginary doorway - if not to another world, but at least to the adjacent room.
Of course, most people know or would readily believe that great stories transport them to another world or another time and place. But what this research, coupled with my conjecture, could suggest is that it is a cognitive trick related to memory - and in particular episodic memory - that creates that illusion.
It also suggests that certain key phrases or lines serve as subliminal commands - in effect, setting up, or even resetting, a new memory episode in your mind. Often, without your knowledge.