At this stage of my writing life, I tend to favor writing in the first person. Further, in my current series I have four main characters. Each character gets a turn as narrator. The commonly referred to problem with multiple narrators is how to make each narrator possess their own character. In other words how do multiple narrators each sound unique when they are written by the same person?
One factor, and yes there are many, has emerged that has helped me frame each character and further their own unique voice. It likely has an official term, but it seems to me that children start out self-centered. Let's call this inward-facing. As that child ages that inward looking tendency slowly starts to include other people and concerns. It becomes more outward-facing. As adulthood is reached that outlook on life may well be mostly outward facing.
Kids are all me me me me, right? Adults, most of them anyway, are more concerned about others than they are of themselves, especially parents. If the adult is seen to be self-centered they are commonly thought of as immature.
So in developing my characters, their interactions and their plot lines, I have tried to differentiate each of them by where they are on this "self-centeredness" graph. Youngsters are self-centered and inward looking, completely concerned about what happens to them and what they want to do. Adults and parents are more concerned about the welfare of their kids and less so about themselves. More outward facing.
Where do grandparents lay in this graph? Where do friends fit? While this "self-centeredness" aspect is not perfect and not always consistent, it does serve as an interesting thought that I deploy to help differentiate how each character talks, thinks and interacts with those around them. In this way I am aspiring to give each character their own memorable and distinct character. Their own voice.