When I am in writer's forums and groups, I see concerns about how to write believable characters.
One of the exercises I like to employ is writing in fanfiction. Every once and awhile, another person's universe intrigues me. I have written for Babylon 5, which is online. It needs a good polishing but I can still live with it.
In that universe, I can play with characters. I can use the already made universe to stretch my writing muscles without having to do all the heavy lifting. I can see how close I can come to recreating the characters of someone else.
I make note of my thought processes. What do I need? What am I missing? How close can I get?
I then take what I learned trying to mimic and apply those lessons to my own creative world building.
I implement the same brainstorming ideas to develop original characters in a universe I created from scratch.
Writers can use any character out of literature or film they wish to try their hand in character development.
That is one exercise, but character development is a more involved process than one exercise. To make three-dimensional characters, a writer has to take the time. Write a description. Give your character some real life quirks. Don't make your characters perfect.
And, have fun with your characters. Show their sense of humor, pet peeves, phobias. A character should reflect the time and place s/he is being written for. So take different aspects of people and a little of yourself, a character needs to be built for the task at hand... make your character worth the read.
Character development does not have to be a dirty job. Create exercises that appeal to you and brainstorm a new character. When a writer takes the time, characters can virtually write the novel themselves.
I actually prefer a feisty and spirited protagonist... it makes writing less complicated.
Have a great and wonderful day!