Another challenge to new writers is point of view. No matter how many books you may have read, it does not necessarily stand out what this comprises until you’re confronted with it as an author.
So what does it mean? Generally speaking, everything, including all narrative, needs to be that as seen through the point of view (POV) character’s mind and eyes. This includes the vocabulary. If your protagonist is a child, don’t use big, complicated $5 words unless s/he happens to be someone like Sheldon Cooper on “The Big Bang Theory.” If you’re writing an historical novel, keep the terminology and vernacular, including any euphemisms, accurate to that particular era.
If he or she is a professional, then they should filter their environment and situations through those particular eyes. For example, if your protagonist is a psychologist, he will see things slightly differently than an engineer. When I was writing “The Terra Debacle: Prisoners at Area 51” I had to get into the head of an astrobiologist. This meant I had to learn a whole lot about biology, lab operations, and so forth if I wanted to keep the story authentic. This is what research is all about.
If you read the posting the other day about character building, I’m sure you can see how this contributes to that as well. Everything your character says or does contributes to his personality. I you can’t get inside your character’s head and know these things, then you don’t know him or her well enough yourself.