Tips on Writing Proper English

Fiction writers have a lot of choices to make with every line they write.  Whether it is science fiction, romance fiction, or suspense fiction the rules of the English language have to be followed.  There are of course some exceptions.  We want our characters to be realistic and authentic. Let's take these issues one at a time and try to accomplish some great literature when we write.
Novels require a great deal of thought.  Suddenly you come to the word OK and you have to decide whether to write it as O.K. or okay or OK.  It certainly may seem to be an insignificant detail but this type of thing can stall the writing process and make a good writer stumble.  What is the rule?  I will tell you what I think but any popular book may contradict me.  I asked this question to three people and got three answers.  If it was me writing, I would simply write "okay" in any normal passage in a novel.  Now, this may not be true if someone was speaking - I could easily see the use of O.K. in a quick response to a question.
And when we have to write a number, do we write "five" or 5?  I always prefer to write the words rather than the numbers - up to a certain point.  Personally once we reach one hundred it seems silly to keep using words.  But up to that point I always write the words of the numbers rather than the numbers themselves.  It's my personal preference.  A writing teacher taught me that way in high school and it always stuck with me.
What about mixing reality with fantasy?  That seems to be a silly point to consider, since we have to base things in reality if we want the reader to see things as probable.  Even Star Wars and Star Trek have kept basic laws of physics based in reality.  A new language can be invented and it can be completely fictional and made up.  Anything is possible in writing.
How do we create a character that is realistic and believable?  I think the key is to have each character clear in the mind of the writer.  Even fiction requires consistency.  When someone wants to read a novel, they want to be whisked away to an alternate reality and do not want distractions like inconsistent writing to distract them.  Get your character clear in mind and make notes of their physical characteristics.  What are they like?  Honest?  Reserved?  Outgoing and gregarious?  Tall, broad, slim, short, or fat?  Facial hair?  Slender arms and long legs?  What is remarkable about your character?  What makes them memorable?  What does their voice sound like when they talk, and do people listen to them?  How do they dress and comport themselves?  Once you make a list of all these details it might do you well to write them down someplace convenient so you can refer to them when you need.  Each character will bring a new description and depending on their use, you may need to remember details for later.  Develop a character that is memorable and realistic.  Think of people you know and what is remarkable about each of them.  Use real qualities and personality traits; and exaggerate them as needed.
English is a very specific language and readers of books need to focus on the story and not on the use of language.  It will require more work from the author but will be work that is worth the effort.