As a GM, how much leeway do you give players to go off the rails?
A lot. Most of the time I'm only a step or two ahead of them.
I have seen some parts mentioned here on the forums. One example is the players taking over the shuttle in Escape from Mos Shuuta.
I wouldn't call that "off the rails" at all. Mind you, there's a star destroyer in orbit which will probably be on alert sooner if the shuttle is hijacked rather than the Krayt Fang, but if that's what they want to do, let them. That said, if the purpose of running the beginner game is to have a learning session, I'd have no trouble just explaining that you're not prepared for that, but in future they'll have more options.
A classic 'off the rails' trope would be killing Anakin before he goes pod racing, or killing off Luke before he see's the Droids.
If the GM decides to throw in canon references and NPCs, they have kind of opened the door to it. Still, it depends on what you want to achieve. You're the GM, if they are lining up a shot on Anakin's pod, and you don't want them to succeed, well maybe those Sand People think the players are taking their spot...or maybe Qui Gon shows up, with 6 force dice, defecting everything automatically, and Adversary 4. There are myriad ways for you to avoid whatever you want.
If the GM doesn't throw in canon references and NPCs, like progressions said, I don't know how the PC would know enough to decide to do those things. The player is no longer roleplaying, and I'd have to wonder why they're at the table. But if the table agrees, and they want to flip enough Destiny Points so they know enough (who and where the Lars family is, and that Luke insulted one of them the last time they stayed there), then fine.
And after it's over, I'd probably have Obi Wan show up, with 6 force dice, defecting everything automatically, and Adversary 4. TPK and begin again. But that's just me, since I'm not into murder hobo canon changing games.
Edited by whafrog, 12 February 2014 - 10:28 AM.