Folks had mentioned they'd like to see notes on converting Call of Cthulhu adventures to Trail of Cthulhu adventures, and how I converted one of the recent scenarios I ran, "Mister Corbitt".
As a preface, I think it's valuable to take a step back and look at the philosophical differences between the games. In Call of Cthulhu, most attempts at information gathering require a skill roll - a spot hidden check, a social check, a read language check. Trail takes a different approach, categorizing information as either a regular clue or "core clue". Core clues are necessary for the investigators to move forward to progress through the mystery, so there's no roll - they automatically get them if they have the right skills.
A scenario can be diagrammed like a flow chart, and a flow chart is basically a dungeon - like these examples: mapping the investigation like a dungeon. In other words, a scene or location where something happens is a room, and a clue is the door or passage that leads to the next room.
Let's say there are 3 pieces of information at a murder scene; 2 provide insight into the murderer's identity and will give the players facts to puzzle over, but 1 of them provides information that ties this crime to a second murder, which is another scene possibility. In this situation, I'd treat the clue that leads to one of the other scenes as a free "core clue" and the other two clues would require spends and player choices. (In Trail of Cthulhu, players manage resource pools over the course of the scenario, spending points to use their ability to get nice-to-have pieces of information - thus called "spends"). Of course, I'm assuming here that in future scenes there will be additional opportunities to pick up bits about the murderer's identity.
Once you equate core clues to mean, 'provides the way to get to the next scene', it's fairly easy to convert Call of Cthulhu scenarios to Trail on the fly; as the group encounters opportunities to pick up information, you just answer this question - do they need this to get to the next scene, if so, it's a core clue, otherwise they'll find it with a resource spend. I do recommend converting monsters and key NPC's in advance, just because the numbers are different and you'll want them handy for the action scenes.
If you happened to read our game reports on Mister Corbitt (here is Part One and Part Two) I added a handful of scenes up front to make the investigation work with the campaign frame - police detectives and consultants working for a special crimes division in New York. Corbitt is still a creepy guy spied on by a neighbor across the street, but instead of the neighbor being a PC, the neighbor is a teenage babysitter. A cop investigates Corbitt's place after the babysitter's call, gets dosed with some of Corbitt's psychotropic substances, and gets mauled in the city by one of the interdimensional beings From Beyond that are visible to those using Corbitt's drugs. From there, it had the opportunity to become a case for the players, and they did some preliminary scenes interviewing witnesses to the cop's murder, visiting the morgue, doing lab work, doing some narcotics and botanical research, and speaking to the police dispatcher, all before getting to the regular start of the scenario. Worked out fine.
Here are conversions of some of the mechanical bits:
The Orange Vine
The orange vine is one of those substances that can bring on psychotropic visions of Beyond, and draw the attention of dimensional beings. Instead of a roll, there would be a spend opportunity to see Beyond if anyone experimented (I'd use spends in occult, theology, or art). Seeing Beyond grants +1 Mythos, and requires a stealth check (difficulty +1) to avoid being noticed by a dimensional being. Escaping the vision entirely is a stability check difficulty 6.
Dimensional Being stats:
Hit threshold 4
Attacks - 2 claws, damage +1
Stability loss +1
The Purple Flower
The other nasty bit was the white and purple flowers in the green house that spray a cloud of pollen that forces rapid decomposition of organic matter, dissolving anyone in the cloud - really nasty. I made it a Sense Trouble to notice the flowers moving (difficulty 8) and a Flee or Athletics to dodge the cloud and escape (also difficulty 8).
Here were stats for the things in the house:
Hit threshold 4
Attacks knock down, damage -2, simple athletics check to stay up
Man-Bagari, the Child-Thing
Hit threshold 4
Attacks - up to 3 targets, damage +1
1 grapple, automatic damage each round from bite suck
Stability loss +1
I feel like there's a bit more to say on running Trail of Cthulhu, so I'll get my thoughts together and put up another post this week - like how do spends and general abilities work at the table, how did my players do with the system, is it still frightening when the players have more control on their successes, that kind of stuff. I'll be back with part 2 later in the week.