Knowing a lot about your character before you begin writing "officially" is a great help here. "A lot" varies from writer to writer - I need to know my character's base level trait. Are they honourable, brave and so on? Other authors write full biographies. I need enough to get me started and then I love to find out more details as the story comes together.
The classic sign of a character who has taken off well is being able to thnk of any circumstance and know automatically how they'd handle it. Also you would know how that character would speak and likely educational level and so on. Let your characters take off! You are the conduit through which they share their stories. I know that sounds a bit "luvvie" but it is true and after all it is the character's story. We want to hear their voice not the author's!