Ever heard of a character being called a “Mary Jane” or “Pollyanna”? Essentially these are characters who are simply too good to be true. They never stray from the right path. Never question authority. Never do anything wrong. They are perfect.
In other words, they simply aren’t real.
Every person has done something or said something they shouldn’t have. Of course, the level of that infraction will be determined by who the person is. So someone we might label a “pollyanna” might have more minor “infractions” like spilling the milk or forgetting their manners, whereas someone with a more checkered past might have more significant troubles.
If readers are to relate to your character, they must be true and believable. They must face challenges, have flaws, make decisions that may not always be the right ones.
To make them believable they must have a value system (whether it is right or wrong) and act accordingly. This is particularly true when creating a villain. Of course we are not going to root for them, but they must have a reason, a goal or desire for what they are doing.
Similarly, heroes and heroines must have a personal value system and a reason for why they are doing what they are doing. It can’t simply be “because it is right.” Perhaps your hero misses a big game and his teammates are all angry at him. He needs a real reason to have missed the game – being home sick won’t cut it, neither will a simple he didn’t feel like coming. Now think about what might really matter to the hero, what concerns him so much he’d sacrifice anything (even the Big Game).
One more important note about believability, once you create a value system for your characters, you can’t just disregard that when it is convenient for the story. T’s from there that the internal conflict and tension in your story builds.
For instance, let’s say your main character never, ever goes on an amusement park ride. He can’t just one day hop on and hop off as though nothing happened. There must be a reason why he’s going on the ride THAT DAY! Think about it. What could be motivating him? What is he hoping to prove, either to himself or someone else.
YOUR HOMEWORK: Create a scene where you character specifically does something that goes against his/her value system. Challenge your writing and your character. Dig deep to make their decision realistic and reasonable.