I think that being realistic is setting up strengths and weaknesses for your character and to stay true to those. After all, this is a Wizarding RPG so it can't exactly be "realistic". For example: My character Carigan is good at divination because her mother tells fortunes for a living. SHe is also good at making things with her hands because of her father. She s good with plants because of where her family lives; however, she has never tended magical plants before so she is a bit hesitant with herbology, but in the end does an okay job.

You need to stick to your strengths and weaknesses otherwise your character is too perfect, and that's not fun to play. If your character is shy, they would probably have more inner monologue than speaking, and vice versa with outgoing characters. ANother example from my experience is: My character Carigan is very outgoing and loves people so she talks A LOT. She also rambles. I will often use what seems like run-on sentences when she speaks because of this. I will also use ellipses (...) when she pauses.

That's just a few things from my own experiences that I find to be helpful.