Player Characters should feel that they, not you, are in control of their actions...
I was playing Traveler with a GM I had never played with, who I will call "Bill." Bill told us that the ship was damaged, and the goal was to reach an escape pod and abandon ship. I was assigned a character that was normally played by a person, I forget her real name, I will call her "Jenny."
We played through the scenario, had a few combats, and then I told Bill that I wanted to go to the control room, and save the ship. Bill said, "Jenny wouldn't do that with her character." Bill then told me I must proceed, as per HIS PLAN, to the escape pod and abandon ship.
Perhaps you can imagine how frustrating this experience was for me, it was a classic GM move called railroading." NO ONE likes to be railroaded. DON'T DO IT!!!
The opposite of railroading is allowing the players to do what they want, even if it derails your plans. For some DMs, it's hard to allow players that kind of control, but trust me when I say, what the players do, where they go, and what goals they attempt to achieve should be theirs to decide. You can offer guidance, in the form of adventures for them to complete, but if the players chose NOT to complete a certain adventure, then DO NOT FORCE IT.
You can ALWAYS build the railroad, put the train on the tracks, even give away free tickets to your players, but you cannot FORCE your players to get on board and ride to the destination you have pre-planned.
Why Railroading is Bad
If you FORCE your players down a pre-determined path, they will realize that their choices don't matter, and they will not want to play in your campaign. People play role playing games so that they can have choices. Rob players of their choices, and you take away the main reason people want to play.
Example of Avoiding Railroading
One time, I had a wonderful adventure planned. The players were supposed to save a halfling country from an evil duke who was invading their land. The players decided, "This is not what we want to do," and they walked away. So, I allowed the chips to fall where they landed, and because of that player decision, the halfling land was conquered by the evil duke, and turned from an agrarian paradise into an industrial wasteland of factories and pollution. And it made my campaign world better as a result.
How to Avoid Railroading
How much time it takes to implement this idea? It's not a matter of time, it's attitude. You have to be willing to put four-eight hours into an adventure plan, and then allow your players to walk away from it. Trust me, this is HARD. But don't worry too much, unused ideas can always be reused at some later time. Just don't tell your players you are doing this, because if you do, they will still feel railroaded.
One technique my brother Stolph uses, Just-in-Time Campaign Building, minimizes the temptation to railroad by not investing a lot of up front time in deciding how you think the story should play out.
When to Implement this Idea
If you are railroading your players, you are angering them, and making them resent playing in your campaign. STOP NOW!!!
It's too late to implement this idea when resentment has built up and your players no longer want to play. If this happens, approach your players, tell them you made a mistake, and promise to do better in the future.
Importance of Idea
Rated 1 to 10 (10 being most important): 10. This goes along with the idea that you are a "servant" not the "master
" of your campaign world. When you railroad your players, you are "mastering" in the worst way possible. If you are doing this, STOP NOW!!!