Keeping it real.  I touched on different aspects of this in a previous post (research, dialogue, etc.), but let's look at a couple more areas where this applies.  First thing to remember is that readers are smart.  They pick up on inconsistencies.  My daughter read an early copy of my mid-grade book The Case of Secrets and found something in the first chapter that didn't jive with something in the last chapter.  Yeah, she's sharp...like her mom.  Some other things to watch:

Words - make sure your character is using words he/she would actually use.  Obviously, these are based on age, occupation, location, situation, and so on.  If your character is 9 years old, is she really going to use the word "pontificate?"   Is grandpa really going to use LOL and LMAO in a text message?  For that matter, does he even know how to text?  If your character lived in the 1800's, be sure he's speaking and writing in the language of the day.  Hint: It was much more eloquent. 

Choices - Does your character make choices that fit with how you constructed them?    For example, the neighborhood bully isn't going to be opening doors and tipping his cap for people.  Well...not until that near-death experience with the psychotic doorman at Vampire Hotel  makes him see the error of his ways. 

Sequence of Events - This goes without saying but let me say it anyway.  Pay close attention to plot sequences in your story.  The more complicated the action/event, the more you need to know how it really happens so you can describe it accurately.   Take blockbuster author Lee Child, for example.  When Jack Reacher gets into a fight, the author not only describes Reacher's physical moves, he also describes the resulting effects ( in accurate physiological terms) of those moves on the challenger.  Or, for those who make the mistake of taking on Jack Reacher, the ones that end up incapacitated or in the hospital.