I have run across discussions what characters do on their own initiative, and notice that my own experience is different from those I have heard described.
Other writers describe their characters doing things the writer didn't intend them to do, and even arguing with them.
Now, my characters are less forthcoming. They do not talk to me. They are, in fact, embedded in their world so that issues with anything from the metaphysics to the motives may come out the same way. Generally as inability to move forward on the story.
True, my characters do act in ways I can't predict. Such as one character, thrown in on a whim to enliven a journey who calmly took over the role of love interest from another character. Or another who was uncommonly nasty in teaching the heroine.
On the other hand, while the new love interest wouldn't even hear of a love triangle, the teacher could easily be revised down.
And when I looked at the old love interest, I realized I could merge him with another planned character, and they amalgamated with each other so thoroughly that only my memory bears any trace that they were separate.
Sometimes the characters are blandly colorless, and I have to assign them some trait to pep them up. It is not unknown for them to take it and run with it, so that background characters suddenly become minor characters, and reappear again and again.
Though perhaps having gaps. Characters have nothing about their schools, or tutors, or which applied, whether they have brothers and sisters, what the ballgown in the wardrobe looks like even though there is a ball coming up, and she would wear it.
And then there are the times when the characters refuse to do the next thing. In the outline I usually decide that the exact opposite happened, and that gets them going. Oh, you were going to go there and consult a sage, only I can’t outline it? A dragon flies in and burns things up and makes everyone run, and so instead they must deal with the new place that they landed.
Sometimes the problem is that I didn't have an idea for a story1. Once I unleashed a character on the world with a secret power that would enable her to reveal what was wrong with the world, while in a snarky first-person narrative2.
What was wrong with the world? Well, that was my bailiwick. She just had the power to reveal it.
But once I had the idea for her love interest, all the rest piled in after it.
Then she told me that she wasn't telling that story in first-person because snarky didn't suit the horrible elements, and she was doing too many wonderful things. Even if she had to do them, first-person did not suit them and her.
Ah, well. That work is still in progress, but at least it's clear forward. Except that new characters are appearing in support, so I'll see.
You Start With What You Start With
I've run across one too many articles and how-to-write books that tell me what to start the story process with.
In My Beginning Is My Ending -- I Hope
You have it in hand, the definitive beginning, the interesting and fascinating way the hero sets out on his journey, or perhaps the wonderful setting that he grows up in, with potential for inciting incidents, or the in media res moment that you think really should be the beginning, with everything else being relegated to backstory.