I read somewhere that stories should follow a theme, which helps keeping the story on track and develop characters. A theme is usually a word describing something that is the result of actions/thoughts of the characters in the story, for example:
Trust, this could be in a story where the main character has trust issues, s/he could be put in several situations where they have to trust others. When they do start to trust others often enough, throw in a person who shouldn't be trusted to make the character go through an emotional journey, in the end they should have grown quite a bit.
You can also cut up the story in parts, addressing several themes. Another way to write characters is to think of Why they do it instead of What they have to do. Example:
Gwen goes into the cave because it's raining Meowths and Growlithes, she finds the Stone of Destiny/Jewel of Destiny etc. because she doesn't have any survival skills at all and doesn't know how to make a fire, and that she is bored (she kicks around the rocks on the floor and sees that one of them glitters). She keeps the Stone because it is pretty, and she likes pretty things. (That and wanting souvenirs, she is on a "vacation" after all.) Later on, another trainer seeks shelter in the cave and rescues her from freezing to death by actually knowing how to make a fire.
Instead of:
Gwen searches for the Stone without a reason, in fact, she doesn't know it exists she just goes into a cave (despite not liking damp and dark places) and looks through the dirty rocks and finds the Stone. She keeps it because it is needed plotwise.
I've found out that if I let myself get into the shoes of the characters, they usually carry me through the story. In order to make them go the way I want, I put a person or object in their way, and if their personalities suit the story, they should go in the general direction that I want. (i.e. While being a very important object, the Stone is also the reason why another character befriends Gwen, and why the bad guys first attacks her. Its removal from the cave is also the reason why another character enters the scene.)
Hopefully this will help you.