I want to write a book about someone who says "I wish I never had been born," and then gets the chance to see what happens if they'd never been born. Sort of like Our Town, only without the person being dead. Any ideas on how I should develop the main character? I have an image in mind, but I'm really not sure how to begin.
well, if the main character was never born, he won't be in the book, other than perhaps at the very beginning, will he?... or do you have a 'ghosts of christmas past' and scrooge thing in mind, where he'll be seeing his family and friends living life without him, though no one will be able to see him?...
Ghosts of Christmas past type thing. I want the main character to see that he is valuable and needed.
Very much like It's a Wonderful Life. But the storyline isn't all that relevant to how you develop the character. Or rather, it isn't very relevant to asking us how you develop a character. Consider what the lessons are you need him to learn, and what kinds of key events in his life you want to affect other people's lives. That should help you decide what kind of person he needs to be. Many people could respond with what kind of character THEY would begin with, but the person YOU choose is what will drive YOUR story.