There is nothing wrong with brainstorming ideas, conversations, random actions, and even characters. However, once the story begins to take form, you need to know your characters. Not just your main characters, but all of them. Ask yourself these questions.
1. What things in their past made them the way they are?
2. What are their basic qualities? Age, style, location, appearance, hobbies, etc.
3. What are they looking to accomplish?
4. What skills do they have to accomplish those goals, and how did they get those skills?
5. How do they react to different kinds of characters? Why?