The backstory… Such an elusive term. What is it? I mean really, when do I use it? I’m not saying this to be cute, by the way, because I have many instances where I’m writing something and the backstory is more interesting than the story itself and I ask myself, was I wrong? Have I been influenced by others to such a degree that I don’t know when my stories begin anymore?
Writing has rules, I try not to learn them, though some are pretty useful, others are more restrictive than helpful.
Some stories doesn’t need a backstory at all! At least not a very comprehensive one while others need more before you can tackle the story itself in a truthful manner; like character motivation and such. It’s all about finding the truth and be honest with yourself when writing.
Sometimes characters do things that makes you confused, you have to find out why he does this rather than have him act unnaturally, because characters sometimes have lives of their own, regardless what you want them to do.
If your backstory is deep, the problem then arise how much you want to show. Generally, you wanna sprinkle it out throughout the story but sometimes you just want to make an entire book about it, like the Silmarillion.
So, again, when does the story start? In my experience, you’re right the first time. The story has a core, the thing that made you want to write the story to begin with. That’s where the interesting part of the story begin so you should probably follow that intuition and go with it. I once wrote a story with three different plots because I didn’t know what I was doing. I knew the main character would end up on a haunted island at some point, but I didn’t have any ideas who he was and how he ended up there. I started the story when he sets foot on the island and the rest should work out from there.