40+ years of reading fantasy novels.
35+ years of TTRPGs/D&D, video games, and anime/manga
30+ years of experience with interactive, short form writing (i.e., online roleplay via chatrooms and forums)
Ongoing exposure to more stories and new ideas. I had been reading stories on Royal Road for only a few months before I was inspired to start my serial.
Narrow that down with a combination of what is consistent with a character's current abilities and their current goals. If a character realizes that they need to figure out a way to deal with flying enemies with ranged attacks, one of them might be focused on finding a way to fly, while a different character might be more interested in ranged attacks of their own, or otherwise making their attacks hit from a distance.