Characters are the most important part of any book. You can write beautiful descriptions, but if your characters don’t pull people in, most people will stop reading. The hardest part when writing is to make your characters do what they would do in a situation, not what you would do.
The best way I have found to make sure this happens is to just back off. I start writing a character and soon it is like the character is telling me exactly what they would do. Sometimes they do things that are surprising and so very perfect that you think to yourself “How did I not see that coming?” To me, that is when I know that I am building a good character, because they are surprising me.