I have experience creating characters I've used for many years who are always developing, and I've made characters for plot, much like how BM usually works. Both styles have their pros and cons. However, for the creation of these characters, I use the same strategy.
I turn off my computer and pull out a notebook. Yes, I could open up a Google doc or something similar, but pen and paper has always organized my thoughts so much better.
First, I write out the plot devices I haven't figured out the details for yet. Often, I prefer RP with other creative individuals that I can trust that we don't need to go over everything in PMs. I enjoy the openness to create, but to craft a character, I need to have my side of plot devices covered, or at least enough to get started on a good foot. These devices will shape my character and the story, and I believe it is important I have some of these aspects meshed out. Sure, things could change or be adjusted, but I still need some clarity as a writer.
Settings, background, powers/limitations, special items, critical events, important NPCs tied to my character, and etc will be covered. What I end up writing on this page really depends on the plotline and RP goals. After that, I move on to the next step.
Then, I start out simple with stats for my characters. Name, age, appearance, and so on. After that, I start to pull from my plot device page to try to mesh out finer details. I also try to craft a personality that will work well or playfully conflict with those plot devices and what my partner and I may like. I also will cover things like a character's fears, likes, dislikes, goals, and other skills.
If it isn't coming to me in a nice outline, I'll free write about the character. Cross out and highlight what I want to get rid of and keep. After that, I reorganize.
I will admit that some of my character outlines are pages long, while others are just a single page of scribbling brainstorming. I guess it depends on how complex the plot is and how strong my inspiration is. Funny enough, if I'm really inspired, I sometimes I just have the scribbles lol.
Once I'm satisfied, I usually don't have to refer back to my notebook much. I do here and there, but when my thoughts are organized, I jump in with confidence I have enough covered.
I also do this outline for very complex posts at times. Sometimes I feel like my head is swimming if doing something like introducing like two nations, attacking, and something supernaturally grand is taking place in one post. It helps me organized it all and try to cover all these details as smoothly as possible in my writing. This also goes for when a partner overwhelms me in a post, I'll break it down into an outline so I can better grasp what is going on and how to respond.
Random note, my least favorite part of characters creation is coming up with names for characters and locations! Drives me nuts, and I'm rarely happy with my choices. After spending more time than I'd like to admit on name lists and generators, I usually get frustrated and settle with something with the hope it will grow on me. Lmao.