hi !!!
honestly I feel like I'm winging it most of the time lol. but I do feel I have to give some of the credit to my interest in anthropology- thinking about human behavior and why human's have that behavior is like. what I do every single day. and also the fact that I've technically been in the utmv fandom for... NINE YEARS?? wow... time has passed.
when I'm writing a character the first thing I do is gather all the knowledge that I have and that exists of a character- as an example, when I first started writing Ink I reread his FAQ a few times and looked at all of the posts about him on Comyet's blog. I also like keeping this information handy- I have screenshots of what I think are the most character defining posts about him, and I always have his FAQ on hand.
I think figuring out a character's motivations is always a good first step. What is important to a character? Why do they find it important? What drives them?
I also personally really like thinking about how a character would perceive the world. I think Ink perceives the world through color- through his ink & lack of soul. I lean heavy on color descriptions when writing Ink, and as a result I'm kinda always thinking about how his ink/paints/vials would be effecting him in that moment. Dream is always thinking about what he can do for other people- being the savior that they need. He's also always thinking about emotions for... obvious reasons lol.
I don't know if any of that is at all helpful lol. but I hope there was something there!!