Know what your character desires most of all.
We go after the things that we want. That’s where ‘story’ lives. Your character is ‘likely’ to be chasing a dream, or an idea in your book.
Make the character more than one dimensional.
This is so effective that screenwriters often use a “save the cat” scene near the beginning of the screenplay to help the audience identify with the character. That character might be a hard-drinking, womanizing, selfish human being but on his way to work he always gives that homeless guy a few bucks for a coffee. Suddenly you don’t hate him so much.
Put a bit of good and bad in every character.
Just as we, as humans, are both good and bad—your characters should be too. Nobody is perfect.
Try to make your characters as unique as people really are.
Some writers fall into the trap of having characters that all sound alike. Try to give your characters a unique voice. If a reader can’t distinguish your characters—how can they enjoy the story?
Give your character a life that lives on outside of the story.
This is one of the most interesting parts of character development. You are creating your characters for a book, or story. At the same time you are trying to make them as rich and colourful as living human beings. Give your characters backstory, likes, dislike, opinions, views and purposes. If you flesh out your characters people will start to see them outside of your books.
Everyone has a passion.
Passions are one of the things that makes us interesting as humans. When you meet someone who has an interest you are interested in that person. When you meet someone who has no interest in anything—it gets a little boring. Give your characters a hobby. This is especially useful in the cozy mystery genre.
Give your character a talent.
Self-explanatory really. Your character will be more interesting if they are good at something.
How does your character treat people, and how do people treat your character in return?
This is all about human nature again. Everyone knows someone they dread spending time with. Everyone knows someone they love spending time with. What makes them different? It’s normally the way those people treat us. The way your character treats others, and is treated, just makes them that little bit more interesting.
Know what your character wants the most.
What does your character want? This will change the way he/she treats others, acts, behaves and it can tie in with each of the above points.