If you want your book to be successful, then you need to think about and define the audience you want to attract before you ever start writing. This includes the age group, gender, music they listen to, events that have happened during their lifetime, etc. In other words, if you were writing for young adults, you would use different vocabulary than you might use for adults and seniors. You want to capture the slang they use to express themselves. To make your characters authentic, know as much of them and the world they identify with, and you will be much more likely to keep them interested, which means they will refer your book to others. A lit bit of research goes a long way. Enjoy!!!