Wedgits
Growing up as an only child was, at times, very lonely. One thing that kept me almost constantly occupied were these blocks called Wedgits (pictured above). Now Wedgits aren't your normal building blocks, in fact if you know what you're doing you can actually use them to create geometric sculptures, robots, etc... and they hold themselves together!
Now that i've completed my Wedgits commercial and thoroughly confused you let me get to the point:
Your characters are like Wedgits.
"What the hell Vira? That makes no sense!"
Hear me out: See those white diamonds? Those are your main character's history, their story. You can see some of the little white diamonds throughout the sculpture. In total there are 10 hidden away, holding the entire thing together. The large white diamond is your character's main history, and you may not see it for a long time, but it's in there. Back behind the blue and green Wedgits, holding up some of the other little white diamonds.
The green? Those are your secondary characters. Their history, while still extremely important, isn't quite as prevalent, contained in only 6 Wedgits. Like your main character's story, these 6 Wedgits are still vitally important, and support a majority of the sculpture.
The blue? They are your antagonist's past, their motivation. Again, while vital to the structure of the sculpture, you see less of them. Coming to a whopping total of 4. But without these 4 Wedgits the entire thing comes crumbling down. Your white diamonds and green squares have no place to be wedged into.
The red? That's your world's history. Whether you have to create your own world, are writing fan-fiction, or even non-fiction, the history of the world you are placing your story in is the base of your story.
Oh, and all that yellow? That's your plot. It looks like it takes up a majority of the sculpture, but surprisingly your plot only inhabits as many Wedgits as your secondary characters; 6. Of course,as you well know it's pretty important.
What I'm trying to explain is this: Your character's history will overlap. And it will change each character differently.
"The same boiling water that softens the potato hardens the egg. It's about what you're made of, not the circumstances."
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Example:
Thane and I have three characters that lived together for a time, but were then separated. One is still trapped by the past, one longs for the past, and one is haunted by the past. For now we'll call them A, B, and C.
We knew there was a reason C didn't trust A and B, we just didn't exactly know why. Then 3 days ago Thane texted me an excerpt from a scene featuring character C and suddenly it all clicked. B is so caught up in the past, in what was, what could have been, that B fails to notice the signs. B's love and compassion for A is so deep that B overlooks what A has become. C was with A longer and saw what happened. C can see the warning signs of what happened, and therefore doesn't trust A. C also doesn't trust B because C understands that B's love for their friend has blinded them. C believes they are the only one who can see through all of the pain from all of their pasts.
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The overlaps in your character's history will always shape your plot. And when I say always I mean ALWAYS. Even when you haven't figured out what the outcome of that overlap is.
Your plot, like your characters, is fluid. You do not have to have the same end goal as the one you started with. Hell, you don't have to end up with the same book you started with. The key is learning as you go, writing and building as your characters grow and mature. Their adventures will change them, and every time it does you add another Wedgit to the tower. Till finally you have the finished project.