Nothing can create conflict for your characters like good old-fashioned fear. Take time now to define your protagonist's biggest fear. Is it something physical (e.g. tight spaces or flying in an airplane) or internal (e.g. fear of failure, commitment, or rejection)? Write a scene in which your protagonist must face this fear.
It's impossible not to put some of yourself and your own life into your writing. But when you're writing about characters who you don't share much in common with, it can be tricky to authentically capture their "voice" and point of view. To develop this skill, fill out this character profile and base it on yourself. Then fill out a second one and make it as different from your own as possible.
If you're struggling with one of you characters, try writing a letter as your current character to their older or younger self. Think of what they might want to tell their past or future self, this might help you pinpoint what's important to your character.
In order to dive deeper into your character's emotional depths, ask a round of questions - both probing and seemingly innocuous alike. (Hey, you never know when your character's favorite choice of ice cream topping might come in handy!) While we encourage you to build and refine your own set of questions, these questionnaires will provide solid inspiration for now: Arthur Aron's 36 Questions That Lead to Love, and The Proust Questionnaire.