You need the reader to connect with your characters. There are lots of ways to do this but if they don’t connect to your characters, they won’t care about your story. You know I’m right. Think of your own reading of books or watching of shows and movies. Sure, an interesting story is important, but if you don’t care about the characters, you’ll give up on the story.
The way to get a reader to connect to a character is through the things the character wants/needs/desires and the things he or she fears. The acts that the character does in order to get what he or she wants and in order to avoid what he or she fears create character. The character’s reflection on what happens because of their actions shows the character’s interior life.
Also, when you get this action & reflection down, the right amounts of each, in main characters it will drive your plot.
Thinking about this in early drafts might help you decide what happens next or how a scene should work. Thinking about this in later drafts might help you select what you should keep and what should be cut.
And that’s that for character building.
Not really. There’s always more to writing fiction. But it is a good start.