A reprint from a recent posting I made, Blogged for several reasons.
And just to elucidate, once and for all, my POV on Plan and Purpose, Railroading and Sandboxing...
(The following may come off as a bit strident, but after some of the snide comments made at my daring to assert that my campaign has a Plan and Purpose, I'll have to ask for your indulgence.)
When I run a campaign, my players are free to do whatever they wish. The world is always going on around them, presenting them with several choices, and if they do something utterly unexpected that's fine. I'll DM on the fly, anytime. I won't MAKE them do anything. I may entice them. I might, on occasion, back them into a corner (After the divine intervention mentioned above, could the player or party Really refuse the god if he expected a favor in return?) They make their own plan; I make mine. Every NPC, every event, every tree and every deity are under my purview. That plan evolves. An important NPC may have half a page or more of background, personality, psychological characteristics, etc., but any NPC with any significant screen time at all WILL evolve, in the direction I so choose. The idea that the DM is some kind of neutral observer is absurd, unless your world is some static museum, or one giant Hack & Slash extravaganza. I have a plan for that evolution, and that plan is not static. It is every bit as responsive to the needs and desires of my players as their plan is to mine (and each other.) There is a synergy here, working on several levels. There's your Plan. The Purpose is to manifest that Plan within the imagination of the group; all for fun, creativity, and love (I see a good RPG game as an art form, an therefore DO place it well above Monopoly, etc. ) As for any application of Rule 0 you care to mention, I'll use it if I feel it's in the best interest of that Plan and Purpose. Furthermore, without claiming perfection, I'm utterly confident in my ability to do so wisely.
(More links next post, including the AKS Hexmapper.)