Writing a novel plunges a writer into another world. It demands that you become so intensely immersed into the lives of your characters, and I mean all of them, minor and major, that they become living, breathing, people to you. If you do this, you will develop your characters fully. You will 'flesh them out' and your readers will grow attached to them.
Writing good fiction is not a formula. Formulas do not necessarily create a page-turning story. What does is vivacious characters caught up in intrigue and crisis. What exactly makes up your characters and brings them to life?
Describing their appearance
Showing actions that reflect their personality
Let's start with dialogue. This is a conversation between characters that gives the reader insight into the characters' lives, personalities, likes and dislikes, their past, future goals, opinions, and concerns. It should flow naturally, but with emotion, whether intense or stoic. Dialogue is the best way to reveal a character's past instead of using either backstory or flashbacks. Watch a movie in your genre and listen to the dialogue between the characters. Listen to a conversation between people in your presence. How did her husband explain why he forgot their anniversary? How did the wife reply?
When it comes to describing your characters, give the reader just enough information that allows them to paint a picture in their mind of them. If you have trouble with this try this technique. Go online and look at a portrait. If your book is historical find a portrait of the past. If contemporary find a photo that reflects your character as you see him or her.
Say you are writing a historical and your female character is a beautiful woman of the Renaissance. She is totally stuck on herself due to her beauty...and possibly her wealth and family status in society. The painting here is called 'Vanity' by artist Frank Cowper 1907. Write down what you see about this woman. Her hair - its texture, color, and length. Her skin which is luminescent and pale. The strings of pearls, how she has looped them through her fingers. Her eyes, how they are looking downward, not shyly, but arrogantly. The curve of her mouth and the tilt of her head. Flesh this character's appearance out and give your reader a strong visual of what she looks like, and how her looks reflect her personality of vanity and arrogance.
Please share your description of this young woman, and let's compare notes. Each of us will see her differently, I guarantee.