If you're struggling with one of you characters, try writing a letter as your current character to their older or younger self. Think of what they might want to tell their past or future self, this might help you pinpoint what's important to your character.
Your protagonist has just been bequeathed $5 million dollars. The money came from an anonymous benefactor who wants your protagonist to donate all of it to five charities. How does your protagonist react? In a short story, write down what would happen next.
What does your character want most? Now consider what he needs most. Is what he wants preventing him from gaining what he needs? Or will he have to find what he wants before he can achieve what he needs?
There's a saying: "Everyone is the hero of his or her own story." For a 10-minute writing exercise, enter your book from another character's eyes. Think about how differently that character would experience your plot and capture that in a short story.
Develop your characters by placing them in a situation where they are faced with a challenge. For conflict inspiration, look no further than these classic moral dilemmas (and, of course, analyze them from the perspective of your character). For an example of a moral dilemma, search "The Trolly Problem."