And, if the players don't follow the "script"? Railroading?
There is no script.
Let's say that I'm starting up a new campaign, and I've played maybe one or two sessions -- enough so that the party feels like they've shared some experiences.
I know that eventually, I want them to fight Orcus.
So if I'm using this act structure to guide me, I know I want Orcus to be the big climax in Act III. It's the focal point of the campaign for me. I want it to be an AWESOME battle.
I know that I can make it more awesome by not having him pop out of nowhere...by making sure the PC's know and are affected by him even now.
The Act Structure tells me that I should introduce the main conflict early on in Act I. Since we've just started, we're somewhere in Act I. So, I throw in a cult of Orcus that they can break up. Succeed or fail, they know that Orcus is a force in the world.
And in Act II, maybe I throw in another cult of Orcus that they don't break up so well. I show them that Orcus has a secret weapon that is totally messing stuff up! Maybe I have them fight another cult of orcus, but make the "BBEG" in the dungeon a wildly level-inappropriate monster who, if the PC's don't run away from, will MESS THEM UP. If the somehow succeed, maybe this is just one of many of these mighty beasts. If they run away, they know that Orcus has these monsters at his beck and call!
And then perhaps they discover a way to bring Orcus out of hiding in the Abyss, so that they can kill him and end his evil forever, by finding a way around his secret weapon! Maybe they gain levels putting down cultists all over the world, trying to stop them from building these monsters, and then, as Orcus's army is finally amassed, a special blessing from a priest they saved forces them to fight in daylight -- the narrative weakness of these beasts! Now the PC's can fight them like tough normal monsters rather than impossible mega-beasts! If they fail, then perhaps the survivors try again later (or perhaps it was just a decoy, or whatever). If they succeed, yay, they've solved the Orcus Problem.
That's all act structure really is.
Step 1: Show the trouble.
Step 2: Complicate the trouble. ("A Twist!")
Step 3: Resolve the trouble.
That's just advice for a DM to follow. Nowhere does it mandate any actions from the PC's at all. All of these things are in a DM's purview to draw on (the resolution doesn't HAVE to be a good one for the party -- the complication doesn't HAVE to be a failure, either). The PC's react however they want.
There's no railroad here. There's no script. It just helps the DM answer the central DM question of "What will they fight next?" in a way that is very narratively satisfying.
And it's flexible enough to accommodate all sorts of changes. If the PC's at level 1 decide they're more interested in exploring the Northlands than fighting Orcus, you can still use your magick DM wand to keep your cool battle with Orcus in the picture by making his cult a cult of Northlands barbarians or something.
It's not a railroad. Honest.