I think this is something we as writers think a lot about - I do anyway. I was contemplating recently a character in my current work-in-progress. He doesn't enter the stage until toward the end of the novel but plays a very important part. So, I was thinking ("a dangerous pastime - I know"*), how do I give him his own voice and make him someone my readers want to cheer for (or despise if he were a villain)?
I need to get to know my character better because I'm struggling with this. What motivates him? Why does he care? What things does he dislike/hate/fear? What is his biggest strength? What is his biggest weakness? Because we know him for such a short time, his growth will be less than that of the main characters, but we still need to see either a way he grows or a way he digresses - because we like him, I'm choosing growth. He will have to overcome an old and bitter grudge and come to realize that he was wrong.
Now, how do I give him a unique voice? That's a dilemma because I don't personally know anyone like him. I guess I need to dig a little deeper. Arrogant, always right, strong willed, justice-oriented (wait, I do know some attorneys...)
It's true, my characters are all bits and pieces of people I know or have known. It's so fun to people watch and use things we do or reactions we have - you know, truth is stranger than fiction...
These are just some of the ways I create my characters and I do hope they are compelling.
*LeFue and Gaston, Disney's Beauty and the Beast