People ask about the writing process a lot - whether it is better to outline a story first or to just start writing and see what happens. I've heard about an equal number of arguments for careful planning and for seeing where you end up as you're going. Here's what I think:
The other day, I told a friend that I had spent the whole afternoon just running scenarios in my head. My friend said that she wouldn't be able to do that - that she would need to write the story down in order to figure out which scenario would work best. But I have a very hard time writing something with no idea where I'll end up. Then I read an article about personality differences in the way that people answer questions: some people start answering right away because the act of talking helps them figure out the answer, whereas others formulate an answer in their head before saying anything out loud. Either method works - it just depends on how you process your thoughts. I think that same principle could be true for writing - for some people, the act of writing might help them to figure out where the story is going, while other people might find it easier to map it out in their head before starting.
What you should do might depend on the way the story enters your mind, and the speed of your thoughts in relation to the speed of your writing. If you're sitting at the computer deciding what should come next, you can either choose a direction and type it out to see if it will work, or you can run the scenario in your head to see if it will work. Whichever way works better for you will probably tell you whether you're more of a planner or if you'd rather just see where the writing takes you. But it is possible that whichever method works better for you is related to your personality, in which case, either way would be correct.