PCs can be unpredictable. These 4 words should be carved in a DM's mind and used to guide him when he makes yet another adventure. On the surface, a good adventure may seem like just something to entertain people, but it actually takes a lot of work before it is created.
Just like the weather, PCs are nearly impossible to predict as nowadays, they are getting more creative. They don't know that you have written only one line of script for the retired soldier they met while going to the local tavern for a drink, and may be FAR more interested in him(What war did you fight in? What rank were you? What was it like?)then in the quest to save Archbishop Kendell's son from a group of barbarian marauders(Ah, that's boring. Not worth the effort.), and may even try to hire the ex-soldier into helping them with their quest!
They aren't unpredictable over only NPCs, though. If you present them with a cave which a horde of Ogres are attacking, it won't help to list down the damage all the items in the cave would deal if used as a weapon(Stalagmite-3 damage. Runestone-1 damage. Large rock-3 damage.)because you know what? In the end they will just make the cave collapse on the Ogres.
But how do you plan out what happens next, in that case? The answer: Improvise, Plan and Prepare. First, you prepare for the obvious routes like killing the Ogres or teleporting out with the new scroll of teleportation they just got. Then, you plan the results. In this case, if they fight the Ogres, there would be an encounter, obviously. Or they simply teleport out of the cave. Finally, when the actual thing comes, and the players to something you never expected(and believe me: it happens.), improvise! Make sure you come up with a result that is realistic, reasonable and meets the players' expectations.
For more information about improvising, see the next few posts. One of them should be about improvising. Happy Reading!;)