mmm, lemme see... i'd reccomend getting the whole plot done first and then try to imagine how the characters would act (you don't have to do this though since im going for accuracy, even tho im not really accurate for fpn since the characters personalities aren't TOO revealed, but what i did was just talk with a few chat bots on c.ai to get a good feeling on how they would act lol)
(if you're making a game,,,im not too sure if this is what you want because im not that good at english surprisingly because its my second langauge, look down a little bit more)
for sprites, you should have a set of emotions for your characters. For example: Neutral, Angry, Happy, and sad. You could add more emotions onto that if you want to though! For music...I'd say you could go and look for someone who's pretty expirienced at making music or just go onto youtube and find some non-copyrighted ones
If you have any more questions, feel free to ask me! I'm taking a 5 week break off of coding, so I'm pretty much available to ask anytime.