While reading On Becoming an Author, I came across this need for a writer and I found it very interesting. Yo me, it means becoming the character you're writing about. This holds especially true for your major characters. A writer needs to step into the character's life and discover likes, dislikes and other things. If not, all the characters come out sounding and acting the same.
Putting yourself into a hero or heroine is usually not hard, or is it? I've read many stories where the dialogue of one character could be said by another just as easily. Are there tricks to learning to live the lives of your characters? There are some tricks to be learned. Choose the character's occupation and try to be around people wo practice that profession. Here's where you learn the buzz words and the attitudes they bring to life in general. Listen to how people speak to each other. Read books on psychology and other aspects of human interactions. Write a scene from one of the character's viewpoints and then do it again from another character in the scene. How do they differ. Tear the scenes apart and do it again.
For me, Astrology often gives me an insight into the complex natures of my characters. What you need to do is become the character, not yourself telling about the character. Showing is the word and if it means tearing up or revising do it. Actually revision is fun since it allows you to add layers to the characters. This doesn't mean substituting one word for another. It means putting the feelings in and in your mind becoming the character you are writing about. I once read a statement that creativity is akin to madness. For just a bit, you need to enter a state of madness.