Do you understand your character's motivation?
Can you take an event from real life and put it in your story? What makes people believe a real life event but not when they are reading it?
I'll tell you. Because, in real life, people see the event happening. They accept it as a part of life. However, if you drop a "real life" event into your story, the reader will not accept the event because they didn't see it coming. The reader doesn't believe your character is capable of the actions you make them take.
You must build your character throughout your story. This gives your reader the ability to understand and empathize with your character. The reader gets to know your character and understands your character.
In order for the reader to understand your character, you must. If you don't know why your character acts like they do, how will you convince your reader to finish reading your book?
"In order to give your characters adequate and believable motivation for everything they do, therefore, you must give them the most typical and logical motivation - the thing most people would do under those circumstances." says Scott Meredith in his book, Writing to Sell.
Sott Meredith goes on to write, "It is only when the character's motives and actions are different from the norm - when he does something for an inadequate reason, or when he does exactly the opposite of what a normal person in that position might be expected to do - that it is disbelieved."
What happens when a person acts like they do in real life is acceptable. Make sure your character's actions are acceptable as well.
The only way to do that is to develop your character throughout your story and make their actions believable to the reader.
If your character is the hero of the story, then build your story around how brave your character is. If, three-quarters of the way through your story, you suddenly show that your hero acts in a timid way and doesn't rescue the girl, or punch the bad guy, then you've given your reader a reason to not believe in your character anymore and they will probably end up closing your book in disgust.
But, if you've developed your hero and shown the character's background as having a moment in their life that identifies why the hero didn't rescue the girl, then your more likely to keep the reader reading.
For example. Your hero wants to rescue the girl, but she's surrounded by snakes. And, in your hero's past he experienced a frightening time when he was trapped in a well as a child and the well was filled with snakes. You show in your story that the hero must fight with his internal conflict of being afraid of snakes and his external conflict of needing to save the girl. When you do so, the actions of the hero then become believable.
"Your motivation, in short, will be sound only if the character always acts typically - always acts like the kind of person you have shown him to be." writes Scott Meredith in Writing to Sell.
Now, let's take your snake-fearing hero. He sees how the girl is disgusted at his cowardice and so he's forced to be heroic. He overcomes his fear of snakes, rescues the girl, and they live happily ever after.
You've made your character act "drastically" different in a way that shows a change in character the reader can believe. You've given your character a way to reasonably manage his conflicts and change his character motivation.
In doing so, you've written a book with characters the reader can relate, believe in, and grow to love.
So, I will ask you again:
How well do you know your character?