So I have a premise. Great. Now what? If I seem to be asking that question a lot, it's because I am.
Lajos Egri (and if I seem to be quoting him a lot, it's because I am. I think his book is one of the best on the subject, and most clearly defines the elements that make up story) writes that plot comes from character. You must know everything about your characters before you can know how they will react in a given situation.
For the first draft, I ignored this advice. I decided I would get to know the characters as I went. It worked- by the end, I knew them well, and that was reflected in my writing. But the first two-thirds of the book showed that I didn't know them. This time through, I decided I didn't want to finish another draft and then have all that work left of going back and fixing the characters.
Because Egri is right. The choices humans make are a direct reflection of their life experiences. My story will be richer and deeper if I can make the characters three-dimensional, with motivations that form naturally from their pasts.