|"I'll build my castle here. It shall be made of awesome."|
Another important point to make is that your characters, in addition to being fun to listen to, must also be fun to watch, and rarely is a character more fun to watch than when they're embroiled in conflict, and ever the more so if that conflict is representative of your character's greatest fear.
Before you set pen to paper, it's a damn good idea to know what your character fears the most. Screenwriter Blake Snyder called these primal motivations: "survival, hunger, sex, protection of loved ones, or fear of death." What, at its most fundamental level, does your character set out to do?
And ultimately, what does your character fear the most? What stands in his or her way? What conspires to defeat them at every turn? If you're a writer, you have to know the answers to these questions.
A character's reaction to his or her greatest fear is to see their flaw in action.
Unsuccessful screenwriter Joe Gillis, in Billy Wilder's Sunset Boulevard, fears a one-way trip back to Dayton, Ohio, where he'd likely spend the rest of his days in obscurity, covering local funerals for the paper. His flaw kicks in when we see how wealth goes to his head, how he pushes away those who love him the most. His desperation to be somebody, his flaw, lead to his actions to pursue success from a questionable source.
Headstrong Jedi Anakin Skywalker, in George Lucas's Star Wars saga has plenty of flaws, the greatest of which we see in action as he commits mass murder to prevent his wife's death. His inability to accept that everything dies (foreshadowed rather strongly at the death of his mother) causes his ultimately doomed attempt to save his wife.
In Christopher Nolan's Inception, information thief Dom Cobb fears never seeing his children again, so he takes on the riskiest mission of his career despite his self-doubt, due to the guilt that dogs him over his own wife's death while performing a similar operation. His doubt manifests itself as the film's primary antagonist, the memory of his deceased wife, who appears at the most inconvenient times while he invades the dreams of others.
Silent film actor George Valentin, in Michel Hazanavicius's The Artist, betrays his flaw, excessive pride, after the birth of talking pictures sidelines him. He stubbornly remains locked in a world that he believes should cater to him, as opposed to the way the world actually works. He refuses opportunity after opportunity, and hastens his own downward spiral.
If you underscore your characters' greatest fears, you will generate motivation for the defining actions they go on to perform in your story. If you set up the odds just right, they'll want nothing more than to prove to themselves, the world, and you, that they can do it.