To be honest I don’t spend a lot of time writing or prepping scenarios. My experience is that the more you attempt to nail down what’s going to happen, the more you’re actually straight-jacketing the possibilities for things to go in unexpected and fun ways.
Let me walk you through my usual method. Here’s how I prepare an adventure:
I usually start by picking something that already exists to riff off of. In my example case I decided to ”remix” a movie I had just watched earlier in the week: The Vampire Lovers.
Basically, The Vampire Lovers is already a filmic remix of J. Sheridan Le Fanu’s Carmilla, a tale of a young girl being preyed upon by a mysterious visitor. (Now that I think about it, Carmilla is already a novelistic remix of Coleridige’s poem Christabel.) Halfway through the process I added elements drawn from the movie Black Sunday just to freshen in up.
Putting pen to paper, the first thing I do is sketch out the main NPCs. Since the player characters will be investigating
the strange goings-on in an ancestral manor house, I made notes on the residents:
Elena Karmore - 16, red-haired, beautiful & innocent, has fallen ill (lamprey like wound on throat, blood on sheets,
weak and comatose)
Morgan Karmore - father of Elena, veteran of a war in the past, stern but caring, will do anything to save his daughter
Dr. William Hull - acts like he has something to hide, powerless to improve Elena’s health
Carmen Delinda - governess, 30s, dark-haired
Boris Norling - hulking & strong, loyal servant of the Karmore family, superstitious, believes the illness to be witchcraft
Lizbeth McDonnel - 16, blonde, visiting the Karmore family, ethereal and stares into space
In Carmilla, it is the young woman visiting the afflicted family who is the predator–she’s a vampire. I decided I didn’t
want to go with another undead villain since I tend to overuse those, so I decided that a witch was siphoning off Elena’s
blood with a strange external organ. But I also wanted to switch it up a bit, so I decided at this point that the governess,
Carmen Delinda, was the culprit.
I also decided to plant red herrings that could implicate any of the above as the cause of Elena’s illness. A search of
the NPCs’ rooms would reveal that Morgan had brought back books of black magic from his time in the war, the doctor had a number of mutant organ specimens in jars of spirit, Carmen’s room was conspicuously bare, and Boris’s room had pagan idols mixed in with icons of the orthodox faith. I also had a list of ways the NPCs would cast suspicion on each other, but these barely came up.
Now that I have Carmen established as the villain, I wanted to figure out who her minions are. I decided on redcaps
because redcaps are creepy as fuck and it makes sense that evil fey would align themselves with a pagan witch.
I then decided that Carmen needed a second in command, so I made up an undead woman whose face is obscured
by a black lace veil who drives a spectral coach. Now that I’ve added an undead creature to the mix, I retroactively
made Carmen a witch who has returned from the grave to seek revenge (this is where Black Sunday as an influence comes in.)
I had also decided that I wasn’t going to make new stats for any of these. My love of reskinning is known far and wide. Carmen is basically an evil cleric, the redcaps are goblins, the woman in black is a ghoul who uses a whip instead of claw/claw/bite.
At this point, the back-story has emerged in my head: a century ago the missionaries who brought the Church to this
area waged a holy war against the indigenous pagans who refused to convert. The ringleader of the pagan resistance
was a witch named Lady Nemarc (yeah, I did the anagram thing, sue me) who was eventually hung by Church inquisitors. However, at the moment of her execution she cursed the town, saying ”We will drink your children’s blood! Our vengeance will wait!” Which means that Carmen has come back to fulfill the terms of her own curse; Elena is to be
the first of her ex-sanguinated victims.
I then sketch out some NPCs in the town who can reveals bits of the back-story as the investigation proceeds: local
priestess, librarian, storyteller at the tavern, etc.
Now I need a hook, but this is easy when the players will go in the direction of adventure: they find an overturned carriage on their way to the village; the coachman bears a lett‚er from Morgan Karmore to the cathedral to the south begging for them to send an exorcist to help his daughter. Then, at the inn, Boris bursts in to recruit anyone he can to help protect Elena at the Karmore house. Once there, the characters can investigate, ask questions, etc.
Then I make a list of things that might happen each day and night; characters being pulled aside by NPCs dropping hints and red herrings, attacks by the redcaps at night who want to remove Elena from her bedroom (which has been blessed
by the local priestess, unwittingly preventing Carmen from feeding in that chamber), etc. These aren't "railroad" elements; they're just things that could happen in response to the players's actions and interests in the game.
Then I sketch Carmen’s lair, a simple faerie mound in which she was buried by the inquisitors who killed her. Add treasure, tidy up, and that’s the framework. Now, there is no telling what the players will do within that framework. In fact, there were some really awesome things that emerged that I hadn’t counted on: the party’s assassin using her disguise ability and a ventriloquism spell to impersonate Carmen to send the redcaps away, a high-speed chase on horseback as several characters attempted to leap onto Carmen’s spectral carriage, etc. But those unexpected bits are the best part. You can’t plan on them, you just need to give them the space to come up naturally.
All in all, I end up with about two pages of notes, which is more than enough to run a three to five hour game with. It's loose, and you have to be willing to improvise within the framework, but it is what works for me.