If you watch or read any modern horror story you will find that you can split the environment of these stories into two broad categories; Open or Isolated. Examples of Isolated environments are easy to find; The Thing, Phantoms, Evil Dead series, the Alien series, Splinter, The Mist, Predator, etc. An Isolated environment enhances the fear of both the characters and the audience since no help is forthcoming once everything inevitably heads south. It is also much easier to keep a story tight and free of continuity or plot problems when there are less variable story elements to deal with. The Open environment type of modern horror tale is uncommon in comparison but offers much more fertile territory for Modern Horror gameplay. Examples of Open Tales would be Predator 2, Stinger, Hellraiser 3, The Nightmare on Elm Street series (yes the characters were isolated in their dreams but they lived in a large town and thus Open), The Fly (1986), Mimic, The Strain trilogy, They Thirst and any Stephen King story set in a small Maine town (most of them). There is the rare exception that operates as a hybrid of these types, Jaws being the classic example.
Another difference between these two environments are the number of survivors at the end of the Tale. In Isolated Tales there is never more than 2 survivors (sometimes none). In an Open environment the only characters in danger from the Horrors are the characters directly involved in the plot. For example, the vast majority of the population of London survived American Werewolf in London. The difference in survival rate means that if the DMs wishes to play many Isolated Tales he should never have more than 2 characters that aren’t Victims. There should also be a stable of characters a bit larger than the number of Victim players so that people can keep playing rather than finding themselves sidelined (though a low character population can prove to be great motivation for wise play and not just throwing a life away) until the next Tale starts.
As a reversal from Isolated Tales, Open Tales offer the Victim players the option to be involved in every scene. If the DM wishes he may allow the Victim players to take on the roles of any secondary characters in scenes involving the Survivor PCs. For example, one player could be playing the a sidekick-victim that is off doing research and during a scene where the other Survivor Protagonists are sweeping a sewer for Horrors he takes on the role of the City Employee guiding them through the tunnels. Another example, would be playing the first victim that always gets offed early in the proceedings (see any Friday the 13th movie) as a means to get the story started and then the player jumps to the role of his main Victim character.
Of course, if a player intends to play 90 year old Granny Smith he isn’t going to be allowed to pump skill points into Climb and feats in Combat Martial Arts. The DM must make sure to list the Skills and Feats available to any playable NPCs. It goes without saying that XPs can be earned when playing any secondary roles (of course I did just say it…).