Sorry for the late reply! Well an OC's character is extremely important and it's hard to keep it in check most of the time. This is usually why I try to put myself in my character's shoes. If you're referring to Max from The Past is a Painful Thing, when I write for her, I try to think, "Okay, if I were Max, how would I act? What would I say? How would people affect me?" When writing for a difficult OC, it is important to keep the character and backstory in mind along with the character's relationships. I hope I've been of help and if not, I'm sorry!