This document gives advice on where to find adventure ideas, and how to tailor an adventure to the player characters. Closely related, is the task of coming up with "hooks" that will lure the characters into the adventure.
Or "Where do I get ideas for adventures?" Note that this only touches on where to start, not on how to structure the adventure and manage all the details. For that, see Designing and Running Adventures.
If you wanted to, you could run an adventure that follows exactly the storyline of an existing novel or movie. You'd have to find a way to fit the current characters into it, but other than that you could have the same villains, same settings, same goals, and (with any luck) the same conclusion.
There are some problems with following that approach, though. For starters, if one of the players has read the same story, it can be problematic. It's also unlikely that the player characters have the same motivations and goals as the characters in the novel. Lastly, it's pretty unlikely that the PCs would all make the same choices and arrive at the same conclusion. Remember the "Option X" rule: Option X is the option players will choose after the GM presents them with options A, B, and C. Players are unpredictable that way and you have to be more flexible than just following a script. The problems mentioned above also apply to canned, store-bought adventure supplements, esp the "what if one of the players has already read it" problem. Canned adventures need to be written for a more general audience and as a result, they might not be well-tailored to the gamers in your group.
You may run off in the other direction: rather than copying the storyline of an extant novel, you could make up something that it completely, totally new and different. The upside to this approach is that you will definitely give your players something unexpected. The downsides are that it's a lot of work to come up with all that on your own, and if you're not giving the players anything familiar they might have trouble getting into the adventure.
In order to make the adventure a bit more familiar for your players, you could employ some of the overtones of similar stories, but with different characters and settings. (Hence, the attention paid to "overtones" on the Victorian RPG Themes page.)
An alternative to the "scripted" and "completely new material" approaches is to take ideas from a variety of sources or even lift whole characters and settings out of different stories and make them elements in your own adventure. You could even make an adventure that's a "sequel" to a previous story. How might things have played out if, after Dr. Frankenstein's death, the Creation re-animated his creator's dead flesh and brought the good doctor back to life?
One of the big reasons for making the Victorian RPG Themes page was to lay out a sample buffet of interesting history and fiction of the Romantic era that could provide material for adventures. Whatever setting you're playing in (medieval, sci-fi, horror), find and read some stories that fit your setting and write another chapter in the story.
Another form of borrowing is to lift tables, maps, and NPCs from other adventures. These can be adventures that you've already done before (an old ally / foe is making a reappearance), or some portions of an adventure from a book or a fan-written one that you find on the web. This can cut down on the amount of work that you have to do.
Players frequently put a lot of effort into their character sheets and you can use the information they've put down to help you come up with adventure ideas.
For starters, any Motivations a character has can be a focal point of an adventure. More on this under "Lures" below.
A character's background / story can be an excellent source of material. You could (re)introduce someone from their past or have them travel to a place they've visited before.
In the Character Creation Checklist we mentioned that every character should have a list of goals of things they want to accomplish or obtain. This list is as much for GMs as it is for the players. Pick a character goal and design an adventure around it. If the goal is a large one, you might even have a series of adventures that lead up to the goal.
The big win of the character-oriented approach is that it's the most personalized to the characters involved in the adventure and the most likely to engage them because it puts them in the driver's seat.
We refer to the character-oriented approach as "Gamemastering the Star Trek IV Way". The thing that made Star Trek IV so much fun was that every character had a moment to shine: Scotty got to design transparent aluminum on a Mac, Spock got to do a mind-meld with the humpback whale, Kirk got to woo the marine biologist babe, McCoy was able to save Checkov's life using futuristic medicine, and so on.
The best approach typically incorporates portions of all the other approaches listed above. You don't have to come up with all the material on your own, you can re-use material from other adventures, but you should have some original matierial to keep things interesting. It's fun to come up with your own NPCs, but feel free to let a literary or historic figure make a cameo once in a while. And while you shouldn't try to follow a wrote script (or coerce your players into following once), you should at least write one before the game begins. Above all, try to include encounters, contests, and combat that will give each PC their own moment in the limelight.
The reason why the players picked a "motivation" disadvantage for their characters is so the GM could use it. Write your adventure with the various motives of the players in mind. If you don't know exactly which players will be at the table, there are some stock "heroic" motivations that will usually work (rescue the citizens from the baddies, i.e.). Look at the Character Motivation page for examples and details. You may need to remind a player that points can be docked if they don't roleplay their disads.
There's a whole slew of advantages and disadvantages that involve the PC having some contact with a (possibly helpful or informative) NPC. If the PCs are completely lost or you're having a hard time thinking of how to draw the PCs into the adventure, have one of their contacts make an entry and help steer them in the right direction. Any of the following (dis)advantages can serve this purpose:
Alternatively, if you use the Rotating GM style of play like we do, the character that the current GM normally plays could be the contact that presents the PCs with an adventure.
As with the previous, if you're having trouble getting the adventure started or, once started, having trouble moving things along, use one of the character's "strangeness" (dis)advantages to get things moving. Examples:
There's a boatload of different disadvantages that seem designed to get the characters drawn into adventures. (Coincidence?) Here's a list of things that can get characters into the adventure or get them into trouble. (You mean there's a difference?) Every player should strongly consider taking at least one of these disadvantages so as to help draw them into adventures.
For example: The GM notes that one of the PCs has the 'Curious' disadvantage and says: "Your curious nature makes you wonder what would happen if you poured the vial of fluid in the cat food." The PCs see the cat grow into 'Catzilla' and that helps them to understand what they're dealing with and possibly who to talk to next. Another example: The GM might see that one of the players has a patron or contact and says: "Your rich uncle sends you a note. He talks about the curious paranormal activity that's been reported at the museum and asks if you know anything about it." The PC(s) trust this person, so they go to the museum and find out some more information there.
Sad but true, sometimes adventures can get mired down, the PCs get stuck, and nothing's going anywhere. Any of the lures mentioned above can help get you out of this rut.
Ideally, the players would possess the initiative and dilligence to go hunt for clues as soon as things get sluggish, but circumstances (and players) are not always ideal. Besides, you don't have to make the characters work for every little clue they find. Everyone will forgive (and possibly even be thankful for) the occasional "out of the blue" scene where a helpful NPC races into the room with a paper in hand that spells out in black and white where they need to go next - You might not want to make a habit of doing that too often though.