Adapting the campaign on the fly

[Heliomance]

I'm about to start running SilverClawShift's horror campaign, having read it and loved it. Thing is, this is going to be the second game I've ever DMed and I really want to do It justice. I know it's not going to play out exactly the same way as the original - different players have different ideas and different characters have different capabilities. My question is this: how do I remain faithful to the spirit and the awesomeness of the original while still being able to handle whatever curveballs get thrown at me?

ION: iPod autocorrect thinks curveballs should be curveaceous.

[dsmiles]

I love threads about improv.

Here's how I'd do it (given about a week of prep time):

1. Read the campaign, adventure, or module you want to run. Cover. To. Cover.
2. Re-read the parts you believe your players will get to in the session.
3. Think about your players and how they typically react.
4. Come up with 10 ways they can solve the problem/beat the encounter/do whatever it is they're supposed to be doing.
5. Then come up with 10 more.
6. Figure out where each of those solutions will lead the party, even (especially) if it's off the rails/map/script.
7. Script the results for each of those solutions.
8. Profit.
9. Repeat steps 2-9 as needed.

Of course, this only really works if you know your players pretty well. As a for instance:

My friend, in his first DMing session with the group, decided to run Sunless Citadel for us. Only two of us showed up me (a fighter), and another friend (a cleric). We went so completely off the rails he was forced to improvise pretty much everything after we met Meepo. We were an evil party, bent on world domination.

Spoiler

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We enslaved the kobolds, slew the goblins, and took over the town as our base of operations.

Good times, goooooooood times.