I'm definitely a planner. I don't plan out every detail in advance, but I do typically outline or at least put a summary of the general plot - beginning, middle, and end - on paper before I start writing the actual story. How detailed my outline gets depends on the story.
The one I'm writing now, The Road to Bethlehem, is a pretty straightforwar d series of events that take place over just a few days, so its outline is short. It's mainly just a summary of the events and some research notes on the setting. I haven't planned it out chapter by chapter; I've just been writing it one scene at a time, always thinking ahead to what will happen next and which guy I will focus on, since I'm trying to keep it balanced between all five boys' perspectives.
With stories that have more intricate plots or time-sensitive details to keep track of, I tend to keep a more detailed outline. For example, Secrets of the Heart had two separate but interwoven storylines that took place months apart and were told out of order, so I did outline that one chapter by chapter to help me figure out the best order to tell it in. Curtain Call had a detailed timeline for an outline because I was working in real life events from the This Is Us tour with fictional events and had to keep track of when everything happened.
I do change my outlines as I get new ideas, but I don't usually change the basic plot points I planned out at the beginning. It's more just adding details that fill in the gaps to help get me from the beginning to the middle to the end. If I get a great idea for far down the line, I'll write it down in my outline so I don't forget, but I don't write actual scenes out of order.
My ideas tend to be plot-based, so I focus on the plot first. Then I work on any fictional characters I'll need. I usually already know which Boy(s) the story will be about based on the plot.