I've done both, but for the most part, I plan the story first. I have one story that is game driven and even in that story I can't help but start plotting things far in advance and then I have to stop myself and let my sims do their own thing. But from my own personal storytelling standpoint, I do like planning things out first then taking screenshots. It can be frustrating because a scene that I had envisioned cannot be done in the game, so I do end up rewriting quite a bit. I usually get the inspiration for an idea, then write out a very rough draft of the story, so I have a vague idea of what the plot is, the characters, and how I want it to end. Then I flesh out the chapter I'm working on and go in-game and start screen-shotting. When I actually play the game, I hardly ever take screenshots, and I have a separate save file for that. My story sims are like my actors, I guess, and I never really do anything with them besides pose them or take screenshots of them.