And then it's time to listen.
You'll come to feel as if you know their pasts. With some, you won't know their future for certain even though you think it's all in your hands.
You'll come to recognize the facial expressions of your characters: the grimaces, the smiles, the lines and folds of skin. What are they smelling in the setting you created? What do they see around them? Why are they pulling you in this direction when you so plainly wanted to go in another?
That's when writing can feel like the best thing in the world; when it's exciting.
No matter how well you've planned your outline (if you have one) or how well you've scripted the story, your characters may have a different suggestion. And it could turn out that they take you on an adventure you never thought possible.