The characters in a story are what bring it to life. If you don't have strong characters who seem as real as your reader's next door neighbors, you don't have a story someone can get lost in. No one cares about your exciting plot twists and turns if they aren't invested in the people that they're happening to. But how do you deepen your characters enough to make them believable in the story?
Spend A Lot Of Time With Them Before You Start Drafting. In Other Words, Live With Them For Awhile.
I never start writing a story until I have a good handle on who my characters are and what makes them tick. If I don't understand them, my characters become one dimensional and as substantial as puffs of smoke. So once I have my shiny new idea, I spend days hanging out with the characters who will populate it BEFORE I ever put down one word of the actual story. I surf the web for pictures of people who look like them and then print them out. I make a profile of each character, the big and the small. I list physical and emotional traits, special ticks, likes and dislikes. I profile them as thoroughly as any FBI agent would profile his quarry. I can't anticipate my characters' moves if I don't know a lot about them.
Create Back Stories.
I give all the main characters and some of the not so main characters back stories that won't make it into my actual story. I need to understand their motivations, what's hurt them and what makes them who they are for the story I am actually writing. I write them out just for my own reference. The more detail I go into the more real they are to me before I start writing.
Let Them Marinate A Bit In Your Head.
I think sometimes it's easy to rush into drafting before you have a good handle on what it is that you want to say. By taking weeks to do my research and get to know my cast of characters, I waste less time rewriting scenes and wandering around on page after page trying to find my way. My characters are already friends of mine by the time I sit down to write and I feel the need to do their stories justice. I'm invested and as a result, my readers will be too.
Everyone's heard authors say that their characters speak to them. What they mean is that they've spent so much time constructing them that they are now a real thing inside their head, a fully formed being who feels as real as any person in this world because the author took the time to really get to know them. So if you are beginning to write-either again or for the first time, don't rush headlong to the page without a good working knowledge of who your characters are and why their story needs to be told in the first place.