Co-op horror

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If the players are mature,

Relying on the players to properly interpret the gameplay the same way you do is leaving yourself to a very small fringe of demographic that may or may not exist. Therefor really not worth the time, effort, or money of full scale production. Co-op is a mechanic fundamentally designed for "fun" gameplay, which is why it is fundamentally incompatible with horror.

EDIT: I made puns and instantly regretted it.

This is the very definition of a self-fulfilling prophecy. Because you do not believe it can work, it won't. Have a little faith, man. Also, co-op is not a mechanic fundamentally designed for "fun gameplay". Once upon a time, video games were a "mechanic fundamentally designed for fun", and I'm sure you don't agree with that anymore.

Also, you always have to trust your audience to interpret your art correctly. If the audience can go through the entire game/piece/movie/whatever not expending any kind of emotional or mental effort, it's not art. Sorry to say but these are incredibly weak arguments, based more around the presiding status quo and unwillingness to progress than any kind of logical thought process - no offense intended.