I have always found it amazing at the number of people there are in this world and yet you will never find two that are exactly alike. Even identical twins have differences. I look at my own sisters and see how different they are. Many people would ask me how can you tell them apart and I would always reply how can you not tell them apart. They move different they act different they sound different their behaviors are different. Even though they are identical twins they are still different.
Our characters must also be different. They must feel different to the reader. This can be done in many different ways. How they talk, mannerisms that they have, how they look, how they act in certain situations. It is very important that we get inside their heads that we know them inside and out so that when we put them down on paper they are still alive that they are not just one dimensional but become multidimensional.
When I counsel I always start with an intake. This is where I find out everything about the person. I find out about their medical history, their family history, their mental illness history, anything that has anything to do with the person. Were they pre-mature, how was the pregnancy, was the mom on drugs the list goes on. We need to setup an intake session with our characters to find out their inner workings then write up our findings. What are their strengths their weaknesses. Can we diagnose them with any personality disorders or mental illness?
Get to know your character better than you know yourself (I wouldn’t recommend getting to know yourself this well it may cause problems). When we really know our characters then we will know how to write about them.