Sometimes I feel like I spend way too much time on details that don't really matter and won't come up, but I've got this compulsion for having reasons and details that make it all hang together in my brain. Like, even if it doesn't make any difference to the planned gameplay, there are certain bits that I have to get sorted out just to be able to think about the scene. And tonight made me feel really good about that, because I guess this is when it pays off. If you have a player roll a super-critical success and you feel like you have to give him something for it, you can make a sharp left turn and reveal information that leads to an entire improvised scene where they discover that that alien technician was actually a disguised droid and still have it all make sense, because you know what's going on and why, even though before now it was all just scaffolding.
So yeah. That went pretty well. And now I have prepped stuff left over for next time!
(Jason Alexander says "Don't prep plots, prep situations", which is excellent advice. I'm not quite as good at that as I'd like to be. I guess what I do is more like start with the sketchy outline of a plot, but then figure out what the situation must be for it to happen that way. And then I've got a situation, so if (when) the actual plot doesn't go that way, it's fine.)