hello anon ^_^ usually my ocs start off with a concept, then i develop the nuances a little later down the line! admittedly, a lot of my kids' personalities are weird combinations of characters i like... but also i think having an idea of their backstories helps building personality tho! like, maybe you can justify a character being guarded because they were betrayed before... or being a ray of sunshine because they always had to boost the morale in their upbringing..... etc etc!

if i find two characters have similar personalities, i think about what they would do if they were placed in a similar situation! sorry to use my ocs again for examples but JHKJAHWDA i think bud and prae are kinda similar, nervous but enthusiastic childlike people. though, if u placed them in a scary situation, bud would be the type to brave through it (as much as they hate it), while prae is someone who runs away from anything that upsets them lol. i think little differences like these are fun to think abt! i think it really helps to use the "same situation, different reactions" method when it comes to differentiating characters even when they arent similar hehe (also dont be ashamed to steal personalities from existing characters from popular media, im def guilty of that lol >_>)

one last thing: listen to songs. omg. if you think "wow this song would be sexy to apply to a character" you have the power to bend your character to fit the song. i swear ive built so many ocs from listening to songs... a lot of my fave ocs have signature songs that i think describes them best, so maybe that might help u too w building personalties!!