Once upon a time, a friend and I wrote a one-act play for a small theater group here in town. After a number of people read various drafts and we revised and polished, the play went into production. I won't go into the details, but the story involved four elderly sisters, their search for a priceless antique Bible, and their efforts to save a small church their grandfather had founded. At the last performance, a gentleman approached me after the show and said "I have a question for you. Why didn't the sisters just give the Bible to the church? They could have sold it to an antiques dealer and had plenty of money to fix the church building."
I stared at him. Why indeed? It was the logical thing for our characters to do and would have solved everything.
And the play would have ended after about five minutes. This man had discovered a major flaw in our plot. A flaw that I and my co-playwright had missed.
A sick feeling grew in my stomach as I mumbled something to my astute questioner and wandered off. I worried for weeks about this flaw -- long after the show ended. I wondered how many people in the audience had looked at each other and said "Huh?" I racked my brains for ways to restructure the plot to eliminate the fatal flaw. I came up empty. Nothing worked; everything was forced.
But then one day it hit me. I did not have to completely rewrite the play! I simply had to give my characters believable motivation. And with two lines of dialog, the whole thing was fixed.
Ever since that experience, I've worked really hard to give my characters credible reasons for their actions. I don't always succeed, but it's always the first thing I look for. I continually ask myself "Why?" Why is my heroine acting so angry? Why won't my hero get off his lazy butt and actively look for a job? It's not that what they are doing is wrong, but I have to give them a reason for doing these things.
Motivation fuels our stories. Make it believable, and your story will engage your readers and carry them into the world you have created. But if it's not believable, someone may tap your shoulder and ask why they didn't just give the Bible to the church.