Ever wonder how to know and grow your characters without adding length to your work or tons of time to your schedule? Use a screenwriting technique — try writing off-screen scenes for your characters.
While evaluating an early draft of a novel, I found a primary character — the main character's fiancé — lacking definition. Sometimes he was weak, other times overbearing. But the story was solid, as were the other characters. In working with the writer, I briefly considered adding the love interest's point of view, but that would have diluted the thrust of the story — the father-daughter relationship, which was being told through the use of a dual perspective.
Since this story called for a separation early on between the main character and her man, the writer crafted scenes for the love interest at crucial plot points, even though those scenes wouldn't make it into the manuscript. This approach took the pressure off the writer to write what she didn't already know, yet when the character appeared, he was solid, still not altogether likeable, but now it's clear why.
Exercise: Consider a current story where you're having trouble crafting a fully realized main character. Go to a place in the story just before where he or she appears, and write a scene for that character based on what's happening in his or her life at that moment. Then edit the scene where the character does appear. Notice a difference?
For more on how to write hot characters, see this from Writer's Digtest: "How to Craft Compelling Characters."