Not every story has, or should have, a prologue or epilogue. Many authors will prefer not to include either, though they can add sparse but essential text that opens and closes their story – especially so with fiction.
When writing fiction for a client, I always ask if they’d like to include either or both, and to date, my offer has been taken up; and, in my opinion, their stories were enhanced as a consequence.
In terms of the prologue, I advise a short, snappy intro that encapsulates the most dramatic part of the story … something powerful that not so much invites but compels its audience to turn the page and continue reading. Around 1-3 pages long, a prologue should be a flavoursome starter to the main course to follow, potentially dropping the reader into the heart of the action, capturing the story’s very essence … the central act around which the rest of it revolves.
It might be a vague hint of what’s to follow, with no clear and obvious connection at that point, or it may reveal something essential about the protagonist or supporting cast (I would suggest the protagonist might be the better selection, though if another character has a pivotal role to play, then that might be a more worthy victor, in doing so providing a red herring that excites, but is ultimately either partly or wholly misleading). It may reveal something essential to the tale, like backstory – a traumatic event, say, that influences a character’s direction and motivation.
As for the epilogue, the word I would most readily associate with it is ‘closure’. Located at the end of the story, it can be a valuable tool if it hasn’t been fully resolved; though, of course, not all stories will have a definitive resolution, leaving them open-ended or suggesting the possibility of a follow-up – another important function in the epilogue’s toolbox.
It can be argued that the final chapter is the best place for a story to be concluded, and plenty of people will favour that approach. If that final chapter has indeed done its job, leaving the majority of a readership satisfied with its denouement (for it’s unrealistic to expect everyone to ‘get it’ – to the contrary, most might not), or as already pointed out, deliberately leaving loose ends that may or not be tackled in a sequel, then an epilogue might be viewed as an unnecessary annexe that fails to complement the preceding story, or even takes away from it by adding a layer of unnecessary confusion.
Some authors will have one or the other, not both. Maybe the story’s opening chapter really hits the ground running, grabbing the reader by the hand and impelling them to jump excitedly into subsequent chapters. Perhaps they might not have found the perfect way to bring their story to a close, in which case an epilogue can add the supplementary material required to really do it justice.
There should be no rules when it comes to creativity, so an author should do whatever feels best – and switch between stories, if adding a prologue and/or epilogue works in one, but not another or others. So perhaps the most prominent message here is: write your best work, and if you feel a little something extra is required to really bring the best out of it, add some polish or throw your reader off in a different direction, then either or both can act as clever ploys to these ends.