"What if all you have is a plot?
This is where I am going to deviate from practically anyone you have ever heard from, and tell you this: try writing fanfiction.
For those of you who don't know what fanfiction is, it's pretty simple, and I would bet that you have vaguely thought about doing something like it without ever realizing it. It goes like this: you see a movie or read a book or even play a game, something you really love, but when you're done the first thing that pops into your head is "But what if they had done---" or "And then what happened?" or "Gee if there had been a character like this---"
Check it out: Mercedes Lackey's pep talk