Personally, my favorite part of any story!
1. Sentence structure!! Remember that how you phrase things i.e. the length of your sentences, the vocabulary you use, the tone you write with, not only reflects on you, the author, but your characters! If your sentence structure is really simple. It could make your characters seem simple-minded, or bored.
2. Try to have a natural flow of events in your story. I know sometimes it's easier to write about only the important events of a story, but the time in between and the reactions to those events is what builds your characters. Try to remember the time frame of your story, and the time it should take your characters to process big events, before moving on to the next one.
3. Don't write an OP character. No, your main character probably shouldn't be beautiful, intelligent, a superhero, have every boy she meets chase after her, be incredibly rich, and speak to animals. Often when you're basing a character on yourself it's hard to give them faults. But every real person has faults, and so every character you write should too! It's much more engaging to read about a character who makes bad decisions sometimes than it is to read about one that is entirely unrelatable.
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