Well (took me almost a day to see this, but it’s in my wheelhouse) for almost ten years I wrote almost exclusively rotating 1st person rotating POV. I liked it because I could control clues to both the reader and the characters. That first cast had really strong personalities, I bet I could write a new scene with their unlabelled POVs and my long-suffering beta could still identify which character was speaking even after five years. A half million words.
And that strong personality is the most important thing. These characters had different moralities and ethics, people skills, slang/formal language usage, faith/lack thereof, careers/specialties- especially if illegal, goals. That’s before you even consider traumas and obsessions and backstories.
Pick a specific actor/part for their line delivery, think how different Robin Williams was for the genie and in Dead Poet Society. Make a familiar character/person to be the core basis and add setting based stuff. Yes, this is all characterization, but that last bit that makes the character gel and become unique and memorable. It binds the fictional universes together, because without chars, the story is just a checklist of events. (stories like that are so painful)
This is why stories heavy in OCs are a challenge, the new characters haven’t differentiated enough to be interesting. I had mental baseball caps I put on the change gears to another character in that first series. I don’t know how I made them the first time, so newer ones are seconds. You need that strong personality and look at the scene/events from their bias and needs. They look out your eyes at the scene you made. You could joke that I have multiple personalities: assassins, paladin, shadewalker, healer, Sith, and Jedi. It would not be that off, and waiting in line causes odd commentary...
Later series weren’t quite as strong. And I’m struggling in my current big story. Original flash stories and other shorts just don’t have as strong personalities they focus on revelation/plot.