I'm actually more accustomed to sandbox. never finished one box set, as a player or narrator. my old group tended to come up with crazy ideas that are so far out, there's no way for the opposition to counter, if they didn't decide to go off and do their own thing. so I stopped planning stories and started creating cast and trigger events(if the go this way, this happens, if they do that, another thing happens) I guess you could also call them trigger scenes. It works for some groups, for others, they prefer a "story starts here" and goes in a predetermined sequence of events, which works pretty well to, if the group likes that.
now some worry about level balance, and at first I did to till I was reminded in one of the books, "life isn't fair, and the game doesn't have to be either" so I went back to building the NPC's to be themselves, even if that left them a lot weaker or stronger than the party (not my fault if they decide to pick a fight with a jedi master when they're a bunch of 7's, thems the breaks) and remember "1's happen"