Characters show themselves.
Do you often find yourself well into your plot before realising you haven’t written any character description for your own record? Planners may well have copious notes about each character’s life history, their personal habits, colouring, hair style and fashion sense. They may have a separate section detailing where each character lives. They may write dialogue including character reactions. They will have detailed his/her personality, the driving force, the key goal and fatal flaw.
Pantsters, like me, will find their characters evolve through having to respond to the action. I don’t know whether I am typical, but I don’t know how a character will react until I am writing that scene. It is like a lived experience. I am with the character facing whatever challenge, embarrassment or dilemma he or she is suffering. They have the experience, they react, I learn what sort of person they are…just as in real life. It is through the scene just written that I learn their flaws. There’s no way I could write the other way around.

Yes, I have tried to plan. The planning guides look so organized, so professional, but for me it’s like being told what to do before I know what tools I will have, or how to talk to someone before I have met them…or perhaps, just being told what to do (by myself, pre-planning) rather than being left free to find out.
And when I’ve written a chapter where the character responds to the situation, I know what kind of personality s/he has, what background experience may have caused the reaction, and I recognize what I need to learn about how the other characters will respond to him or her. It is like an inner source of information that comes (apparently) spontaneously – but in fact will be based on older knowledge of people. This is a different part of the brain from the management, planning one.
This is why I write non-fiction in a totally planned, organized way. Headings, sections, content, sequence. I may add detail later, but it will be clear, that is, pre-ordained, where it will fit.
By contrast, my fiction is character-based. The character dictates the action. Therefore, I know what is going to happen in a later chapter only when I have completed the chapter I’m writing and the character has shown his or her colours.
I do usually have the ending in mind at a very early stage, so in writing the scenes I am a lot of my writing s deliberate creating towards that end. That doesn’t mean I know how the characters are going to get there, or even, sometimes, which characters will do so. One thing I’m determined upon…no weak endings!