Simple Motivation--What Would YOU Do?:
Sometimes to get a plot to work we sell out our characters. We've all done it and it's the quickest way to lose a reader. We coax a character into performing some act that helps the plot but shatters, even momentarily, his character. How can we avoid this error? Give every character common sense. Think about what you would you do if you were in her position?
Usually the root cause of this problem is in the development of the story. We probably developed the character and the story simultaneously. Believe it or not, that's a mistake.
Many professional writers come up with an idea and develop a full story and plot BEFORE tackling character. When the story's finished, they'll develop a character with the right motivation, background, past and current trauma to motivate him to perform every action he needs to perform in the story to make it believable. This technique works better for idea-driven concepts.
Many more professional writers come up with an idea and develop a character to help explore the idea. In this case, the character should be clearly drawn and allowed to take the reader through the story without a writer trying to impose plot points on her. Create a great character and let him go. This works very well for character-driven pieces. One word of caution: if the character isn't fully drawn and well-rounded heading into the creation of the story, you'll have problems, as well. It's a good idea to use a program like Typing Chimp Character to help you explore your character from every angle so you know exactly how they'll react to the story and what decisions they would make before hand.
Either route you decide to go, the most important element for both is to apply common sense. The audience would never question why your character would perform some action when under those circumstance. As a reader, I can put myself in her place and see that every decision, however difficult, makes sense.