The first, is that the "experts" don't even really believe what is happening. That doubt plays into the fear of the unknown, and just overall hopelessness. This is something you can copy and past into any session, because the lack of belief and available help is literally deteriorating several people in the story. Making NPCs doubt, particularly NPCs who should have a handle or knowledge of something supernatural or an aberration in a game is an easy way to keep doors closed for player characters, which is a great way to add tension. Player frustration at in game events has been something I use again and again, and you don't want it always to be the same. The Exorcist is a great source of a variety of that you can throw into an arch in a campaign.
A second great source, is how exactly would a fiend behave towards players? The only real example, at least in possession, would be from the Exorcist. It is basically the standard that every similar film has tried (and I would say failed) to recreate. There is more going on though, as part of the story is about a detective looking into a murder. This is way more obvious in the book, because the film is so over the top at times, I think some people miss it. An unlikely person being used by a being for a murder or other serious crime is a perfect storyline in nearly any RPG. Make that person someone that isn't accessible because of their condition, and you now have a (chef's kiss) easy plot that you can use and adapt, and most of the time players will not even see it coming. This is right out of the Exorcist, and unless you really screw it up as a Game Master, it will work most of the time, and is hard to derail.
The final point about this, is particularly about the film. The movie is extremely atmospheric. The limited set, the creepy subliminal face, the iconic music. Horror in RPGs should be atmospheric. The theme Tubular Bells causes anxiety still for millions of people when they hear it, nearly 50 years later. While you probably won't manage to land something like that for your players, I have pulled off sessions where I really thought about weaving these elements in, and players still talk about it a few years later. I play a lot of games, so I am pretty proud of that myself.
One last thing about the Exorcist, is something more apparent in the book, and that is hinting that some kind of occult activity or a cult of devil worshipers may have had a hand in the happenings of the story. This is something as a GM you can and should jump on, if you feel that you pulled off a successful session. It opens up options down the road if you are planning a long term homebrew campaign--it is always nice to have a few threads available to give you some breathing room planning for sessions.
So yeah, this influence in my gaming is something out of left field, and really goes against what I think of when I think of the place of Appendix N. That is a good thing, because predictable gets boring, especially as years go on and people see the same things over and over again.
I want to make this a regular thing on the blog for awhile, because influences need more love, and people new to the hobby, especially GMs are looking to soak up as much as they can. I started working on a Spotify playlist of Appendix N songs you can find here. As always, emails and comments are welcome.
