OK, a character has popped into your head and you’ve fleshed her out into a full character sketch. Now what? Write a story around the character. How to do that? Well, ask your character. Ask the character about his ambitions. What does your character want? How important is that to your character? Then send the character off to obtain her heart’s desire. Lie to him to give her incentive to start off on the story’s path. Make sure he believes the goal isn’t far off.
Throw roadblocks in her way. Give him the hardest time you can during her journey. Make the character suffer for the object of his desire. How bad the suffering is, is up to you as the author. You have complete control. That’s the essence of a story, conflict, trials and tribulations. Be sure to give glimpses of the prize. Let your characters see a way out, but put chasms in their way. Have your character re-evaluate her desire for whatever it is the character wants. Ask the character again how badly he wants whatever the thing was. Then let her see the goal, just out of reach. Keep the goal just out of reach of your characters. That adds tension and conflict to the story, which drives the plot forward.
Force the character to decide between going on to get what he wants and stopping. Force her to go on whether he wants to or not. Close off the option to go back. Never let the character go back. She started the journey and needs to finish. Rob the character and put him in dire straits. Have someone betray her. Have the villain of the piece be one step ahead of the hero right up to the end. Only in the final moments can you trip the villain.
Once you have your character on his knees, show her a path forward. Give him a little hope. Once your character has heart again and hope that her journey is nearly done. Then take away that hope. Only then can you finish the story and let the character have his heart’s desire.
That sounds harsh but that is the essence of the story. The suffering doesn’t need to be literal, it could just be frustration. The same frustration you feel when you drop something in a confined space and can’t quite reach it. Of course literal suffering is also possible. Put the character into a life and death situation. See what she does.
In either case, make sure you really know your character. Don’t let him act out of the character you initially provided. Torture your character. Alternate between little glimpses of hope before dropping the characters into deeper despair. You can do that and it will be quite legal. Your readers will love it. Remember, you have complete control.