The Horror
Apply that concept to action horror is more difficult than it may seem at first. Let's take them one at a time.
A supernatural force of evil
This is an easy one. Most of action horror has some kind of evil force. The question becomes is it really evil? It could be an animal – a hungry predator – that through its actions can be considered evil. Often, as was the case with Jaws, some semblance of intelligence is attached to animals. This makes them more dangerous foes because it turns them into villains. An animal is just doing what it does best, but an animal that you suspect might be intelligent is now a person trying to kill you. That means it has all the characteristics of a serial killer, but doesn't walk on two legs. And thus we have our supernaturally intelligent force of evil.
Serial killers, who on the surface seem mundane, transform into supernatural forces too. While their abilities might not seem supernatural on the surface (stabbing someone isn't all that magical), to be a worthy foe against well-armed protagonists, serial killers end up getting a smorgasbord of movie-effects, like sneaking up behind people, recovering from wounds, disappearing without a trace, etc. This transforms them from "mundane" serial killers to movie slashers, a breed unto their own.
I have a theory that if a movie slasher becomes popular enough, he will eventually be provided a supernatural explanation for his continued existence. This has happened with both the Friday the 13th and Halloween series, which started out with "ordinary" killers and ended transforming both of them into supernatural murderers, complete with their own cults.
Intra-party Conflict
In a game that by its nature is all about teamwork, it's difficult to work in intra-party conflict. Especially for players accustomed to D&D-style survival, they tend to operate more like SWAT teams than people with their own stupidly selfish motivations. It takes some real role-playing to get a player to make his character perform in a way that doesn't make sense.
This, in many ways, is what separates horror gaming from regular gaming. Horror gamers get that survival is not the ultimate goal; they are instead unspooling and participating in a horror narrative. This is why one-shot games work so well for this horror gaming, because players don't form long-term attachments to their characters and can focus on playing out their characters without the worry of what they will do if their character dies in the next session.
Action horror, on the other hand, straddles the line between the two kinds of play. We're aiming for both horror as well as combat, which means that the players will have to work together if the combat is not going to end immediately in a big pile of dead bodies. But how to encourage party conflict, stressing the characters and threatening their teamwork, without completely derailing the game?
In my campaign, it turned out to be a simple one word answer: girls. Mind you, this is obviously a testosterone-heavy game. But it turned out that a simple flash point I had created between several characters took on a life of its own.
In a previous scenario, Hammer and Jim-Bean saved the life of Dr. Lisa Howell, who just happened to be Guppy's ex-girlfriend. I threw that relationship in on a whim, because it seemed like the nerdy Howell was the kind of girl Guppy might date. When all three players were together again in a later scenario, I let my brother know that Howell had off-handedly mentioned that Jim-Bean had saved her and how wonderful he was. The stage was set.
My brother took it the rest of the way. Guppy kept bringing it up, asking every character they encountered if "it would be okay for one of your friends to date his ex-girlfriend." He blamed Hammer for keeping the supposed relationship, a relationship Jim-Bean denied, a secret. Meanwhile, a giant shapeshifting monster was eating everyone in town.
This tension escalated when the two agents got into a further argument about the nature of the Conspiracy: keeping the mythos secret from the outside world. A scientist, who wasn't part of the conspiracy, was brought in to deal with the monster. When Hammer realized that the monster planned to use said scientist as its mouthpiece to the world, he executed him on the spot.
Building on the tension, that single event triggered a Mexican showdown, with agents pointing pistols at each other. That's when the monster struck.
It's a credit to both players that they didn't just shoot. Both players were mature enough to role-play the conflict without going over the top, walking the fine line between conflict and the fact that we're all there to play together and have a good time. It was my job to jump in with the threat (hello? The monster! Remember him?) and defuse the tension. I of course put Guppy in jeopardy during the attack so that Hammer had to rescue him, defusing the tension even further.
Isolation
In the era of cell phones, Google, and GPS, isolation is hard. Keeping people separated, when the game is all about playing together, is counterproductive to gaming and a lot of work for the game master. And barring an actual dungeon that cannot be simply hacked through, characters have unparalleled freedom to move about the globe thanks to modern transportation. Here's how I create that isolation:
An Actual Dungeon: I'm a big fan of the Cube series of movies, which are basically modern people stuck in dungeons filled with traps. I've run two scenarios with this idea, and the players unilaterally hate it. They enjoy the freeform nature of modern gaming. That said, it's a tried-and-true tactic that works in keeping the party isolated from the outside world.
Other Dimensions: The challenge with other dimensions is the level of weirdness in the dimension. In one scenario, I had characters encountering future and past versions of themselves, seriously messing with time and space. After a certain point, the players thought nothing mattered, because even if they died there were infinite versions of themselves in parallel dimensions. Sometimes, another dimension can be TOO weird.
Bad Weather: I've used severe storms as the premise for a few scenarios, which has the net effect of limiting cell phone communication and travel. If this is in the U.S., eventually this freak weather has to end, and there are consequences to storms consuming entire towns. So I try to use it sparingly. It also requires the supernatural evil to move fast, before the storm ends.
Unforgiving Environments: The old Aliens trick: you've got huge guns but can't use them because if you miss the whole place explodes. Airplanes are a good candidate for this, as are submarines and space shuttles.
Social Isolation: My favorite and the toughest to pull off with groups, this environment is more a mindset. Everyone around the character will not help them. Technology is theoretically accessible but unavailable because the normal modern support structure is actively against the characters, and may even be in place to keep them isolated. Think prisons, mental institutions, and experimental medical facilities.
Achieving all three of these elements requires some planning, but the game master only has direct control over two of them. The rest is up the players.
Lesson Learned: Make sure you have players who are mature enough to role-play tension without killing each other.
