Many of the stories I make have a long-ass backstory. At first I thought it was the main character that needed an indepth backstory, but that's not true at all. He should have some baggage, sure, but the history shouldn't necessarily be explored. All characters deserves a past, so we as writers know what we are writing about, so the characters words and actions speaks true. However, some plot important characters need a more indepth look. For instance, the MC comes to an island (for one reason or another). There's a woman there alone. We must know what she's doing there. This is easy if there's just one life to explore, thus just one story, but in fantasy, it often spawns generations. She's a witch, she has been on the island for hundreds of years, and before her, there was another witch, which had also lived there for untold generations. Suddenly I have another book, or at least another short story to explore. If one should show this backstory is another matter entirely, but how detailed should such stories be? I know I work in waves, like, every iteration becomes more and more complex and I try my hardest not to write anything that hasn't anything to do with the plot before the plot is finished. It's after that I start adding descriptions, scenes and dialogue. Needless to say, it becomes pretty meaty even with my best attempts, and then there is no problem making it into a proper story. The reader shouldn't have a problem understanding the Main story without the backstory and should only serve to bring life to it. But what if one presented the backstory afterwards, kinda like an annex? It seems to me it would be such a shame to hide the backstory I've worked so hard on.
I always made this my main goal. Make sure to present all parts of the backstory that is relevant to the main story. Something I considered. Especially since I read a lot of historical fiction books that do this. I just don't know how much of an interest a reader will have in an annex-snippet if it pertains a fictional world and not our own real one. EG, Druot's Accursed Kings comes to mind where the royals meet a random low noble and the annex explains how that low noble was actually a later reformer in one of the German states and became founder of a long-lasting family line there. Now this makes sense for our world because the more obsessive history buffs (like myself) are vaguely familiar with the name already and this clears confusions. For a fictional world? I'll see after my beta readers comment
The reader needs to know what the reader needs to know to understand the story. They do not have to understand everything. The writer might, but the vast majority of the backstory that you create will never be seen by the reader. It isn't necessary. If the woman's background isn't necessary to move the plot forward, then who cares? Nobody picked up The Hobbit and wondered who dug the Hobbit hole. It doesn't matter.
I've spent many years trying to work with backstories. What I've found is that it really doesn't matter how much or how little backstory you use if you use it the right way. It's all in the delivery. And to get it to work, I think it has to be used subtly and sparsely. You can let the details of a backstory accumulate slowly. I don't think anyone really likes to read chucks of backstory. So, the key is to write it in a way that blends seamlessly, offer it up in bitesize pieces. Don't let it disturb the present narrative, but include it in a way that it is an true component of the story. But this is a case where the writing (the actual prose) is quite important. Again, it's all in the delivery. I don't see backstory as a way to advance the plot or build character as much as a way to add texture and complexity.
It kinda depends what length you're going for. For example, in movies, you don't have such an extended lapse of time to get into detailed backstories, so you need to be more focused and straightforward upon what the story is about. But! if you have decided you're not going for just one movie but for a series (be it a trilogy or a series of episodes), then things change drastically. (Even between a trilogy and a series of episodes that might be continued next year, more seasons, more time). You know from the start though what you are going for. What you're aiming for. I too have a problem figuring this out from the start. Might be a movie, might be a series... Who knows? If I don't start putting the words down on paper, it's hard to calculate even generally a length of a story I'm thinking of. Perhaps, as I'm writing it, I might see that it's building up to be more than I've originally assumed it would. This means that my plan has changed and I have to somewhat restructure and don't be afraid of this word, some things, mostly concerning the depth of the backstories. I already had the backstories in mind before I even started writing, but how much of them and where are my backstories explained differs. This might actually even change the beginning of the story. Knowing that I have more time to explain somethings (because even backstories are stories afterall) I might decide to beggin from an earlier stage of my protagonist's life. It really depends on the time, you yourself give to yourself.
Are there any other ways to introduce the backstory in your particular work? If you want the reader to know the witch's story but don't necessarily need them to know it from the very first page, perhaps it could be incorportated into another plotpoint? For instance, do the witch and the MC become friends? If so they could have a bonding moment over the witch sharing her life story with the MC. Or could the MC, while exploring the island, find an abandoned journal or hidden ruins that tell the story of a lost society going back centuries? I don't really know; I don't know your story well enough. But maybe consider if there are other ways to introduce backstory, rather than just the author telling us in paragraph after paragraph.
