The goal is to make characters real. Even in fantasy and mythical stories of legerdemain and imagination, characters must be real and identifiable to the reader. They don't have to like them, but they have to understand them. Tips in developing full-fledged characters:
- Study people you know. Base characters on them. What can you capture? What stands out? What fits your story? Observe the way they speak, move, dress. All of it creates believable characters. Of course, don't be afraid to create an amalgamation of people. Pick and choose personality aspects. Find what works for you, the story, and the character in question.
- Create a character sheet or bio. A popular technique for novelists and screenwriters. Make a list of characteristics, from physical appearance to likes, dislikes, family members, education, and career, but include personality types: happy and sad are vague, but ambitious, greedy, miserly, philanthropic, gregarious, etc., are not. If he's angry, then angry with what? If phobic, by what? Include details until you know this person inside and out. Keep it in list form or write it up as a two or three page (or more) bio. Your bio sheet can be comprised of snippets and slugs, but it must present a complete character.
- Let characters develop or control them? Some characters simply take on a life of their own, developing characteristics and goals you did not imagine. You can either explore it with them or curtail it. A tough call, with no right answer. Do what works for the story. Generally, you'll want to develop this new trait. There's a reason your creative side thinks it's a good addition to the story.
- Historical fiction and memoir. Two sides of the same coin. Unless writing alternative history, Napoleon can't win the Battle of Waterloo, but you can describe his emotions on the battlefield and his feelings regarding his loss, imprisonment, the French people, Josephine, and his Corsican home. Do your research; don't stretch the bonds of plausability. If writing about the common man of history, don't inject the impossible into a character's personality, knowing what he couldn't know or doing what he could never do.
- For memoir, main characters have to stay true to the story, but minor characters, cab drivers and waitstaff, can be dressed up and made interesting.
A word on implausability. Only dichotemies are interesting. An atheist priest hiding his beliefs from the Spanish Inquisition is unlikely, but compelling and not impossible. A poor Southern farmer helping a runaway slave because his religious conviction tells him to is unlikely, but compelling. A woman on a long stretch of country road picking up a young male hitchhiker is unlikely, but compelling. Examples abound. Just make sure their actions are in keeping with the character you have created. Whatever character needs to be believable is motivation. Maybe creating a compelling character is motivatin enough to write, and motivation enough to keep the reader coming back to your story.