Writing a Mission for Solar Echoes (part 4)

I've written my basic plot, I know generally how I want things to proceed, but the problem is one I'm faced with every time I sit down to start working on a new mission. That problem is that this game is so open-ended: the players can do anything. That is the strength of table-top role-playing games, because there are no pre-programmed parameters (like in a video game) that restrict a player from choices. However, it is also one of the biggest challenges I face, because I have a story I want told and yet I don't want to railroad the players along. Giving players specific mission goals helps, and the experience point system revolves around objectives completed, so that does make things easier. Yet it is difficult to avoid trying to anticipate every player choice and write if/then scenarios for every imagined instance. The thing that has worked best for me is to set forth how each person they encounter will function according to preset motivations. If a bad-guy is fanatically committed to his cause, I'll indicate in the mission that no amount of persuasion will deter him, and that he will do whatever he must to achieve his goals. Then, I leave the rest to the MC—he'll just have to play out that personality in every situation the players generate.