3. Interview your characters.
In order to craft a cast of characters that can help your plot reach its utmost potential, you’ll need to discover crucial details about them, not necessarily at the beginning of their lives but at the beginning of the story.
To do this for your protagonist, work backward from the moment in which he will become engaged in your plot (the “disaster” in your premise sentence). What events in your protagonist’s life have led him to this moment? Did something in his past cause the disaster? What events have shaped him to make him respond to the disaster in the way he does? What unresolved issues from his past can further complicate the plot’s spiral of events?
Once you have a basic idea of how your character will be invested in the main story, you can start unearthing the nitty-gritty details of his life with a character interview. You may choose to follow a preset list of questions (you can find a list of more than 100 such questions in my book Outlining Your Novel: Map Your Way to Success), or you may have better luck with a “freehand interview” in which you ask your protagonist a series of questions and allow him to answer in his own words.