For a lot of other games the adventures are a lot more strait forward. "Travel from Point A to Point B, Fight Monster at Point B, Bring Monster's Head to Point C, Return to Point A for after party". That's a rather simple way to look at it, but it does represent the structure of a lot of games like Dungeon and Dragons. Anything that's really driven by the action tends to be easier to write. I mean, Stats and scene desciptions are easy to generate when they don't offer any real plot impact.
Now, Call of Cthulhu is different in that it's more investigation driven rather than action driven. What does that mean? Well, more or less, every little NPCs and scrap of information to be found counts. Simply walking past an NPC or avoiding a location can drastically alter the information the players gather. In a game where good information is essential to survival, this can be disastrous.
Take for example our first casualty: George Franklin.
George had the unfortunate experience of being overtaken by a Shan, one of those terrible insect of Shaggai. For several years the Shan lay dormant in George's mind after it was subdued with shock therapy until an encounter with The Yellow Sign and the subsequent stress re-awakened the creature. This Shan took the form of an alternate personality, performing night time misdeeds and using George to secretly collect arcane information that would allow it to return to it's world. The clues to all this lay with George's ex-wife in Boston and her new husband, once George's friend and fellow investigator...
Except the party never went to Boston and never discovered the information that would have lead them to believe George had something boggling around his brain stem. Instead they took some initiative and skipped past that bit and went strait on to other George's bit plot. A failled amazing fist/punch rolls later and it looked like ol' George was about to kill the entire party in order to open a portal to the Shan home world! Luckily for the party, and unlucky for George, and swift kick to the ribs ended those plan as well and that character's life.
Since then I feel like my players have done a great job of really getting into the investigative part of the game. They've taken their time, followed all the leads, and it's led them to more decisive victories against the mythos. The plots aren't being laid out in front of them to go through the motions, but the more they play the clearer the threats of the mythos become. Dangerous indeed, but a lot of fun.
The next adventure takes place in New York City, where the investigators have followed their fellow Rex in the hoped of saving him from the ever tempting power of the Mythos. SO, on top of re-familiarizing myself with 1920's NYC, writing up histories, personalities, and stats for Rex's extended family, as well as deciding the general plot points this adventure will follow, I'll have to write up all the information that the players will be able to discover through research, investigation, and putting sole to pavement. Even if they choose to overlook or ignore it.
Another reason I love this game, and I wouldn't have it any other way.