The key for any [creepy/horror] game is tone and pacing - the players must buy in to the tone of the game and be consistent in a more-serious approach than a Monty-Python inspired game. You can set the atmosphere and music and descriptions as much as you want, but if the players crack jokes out of character constantly it will not work and diffuse what you are trying to set up (there is nothing wrong with that in a different game, but not in horror game).
Mechanical suggestions:
-Long term effects that cannot be healed or ended by a short or long rest will cause players to fear certain effects.
-Lingering diseases or conditions (poisoned, exhaustion), curses, loss of Hit Dice, loss of equipment, ability score damage can all increase the terror of the player as they look at their sheet and see their resources dwindle down.
-Have encounters that deal high amounts of single-target damage - in the jungle this may be a hidden crocodile who attacks a single player. If it succeeds, it grapples the player into a river and they must escape as the party decides what to do on shore.
But AS SOON AS THE PLAYER escapes - the croc runs off to find easier play. Treat ambushes as traps rather than full scale encounters - don't roll for initiative and lose tension or the pace of the game.
-Have rests be unavailable, unsafe, or earned. One common thing I see on the boards here is the amount of rests DMs give to their players, then complain about 5e being on easy mode. Running 4-6 encounters, adjusting the difficulty for your specific table, should tax the party's resources and make them crave a respite at the end of an adventuring "day".
Mechanical suggestions:
-Long term effects that cannot be healed or ended by a short or long rest will cause players to fear certain effects.
-Lingering diseases or conditions (poisoned, exhaustion), curses, loss of Hit Dice, loss of equipment, ability score damage can all increase the terror of the player as they look at their sheet and see their resources dwindle down.
-Have encounters that deal high amounts of single-target damage - in the jungle this may be a hidden crocodile who attacks a single player. If it succeeds, it grapples the player into a river and they must escape as the party decides what to do on shore.
But AS SOON AS THE PLAYER escapes - the croc runs off to find easier play. Treat ambushes as traps rather than full scale encounters - don't roll for initiative and lose tension or the pace of the game.
-Have rests be unavailable, unsafe, or earned. One common thing I see on the boards here is the amount of rests DMs give to their players, then complain about 5e being on easy mode. Running 4-6 encounters, adjusting the difficulty for your specific table, should tax the party's resources and make them crave a respite at the end of an adventuring "day".