I think that in order to have true horror you need the player to feel for your characters before anything terrible can happen to them. So in order to do so you would need moments where the player can interact with them or know more about their story and their motivations.
And I don't think voice over is the solution. It's the situation that has to be horrific, so animations, visuals or texts are good enough, and most of the time imagination is the strongest factor. And voice over are tricky, they can make the whole experience cringeworthy if not perfectly done.
Some ideas:
- During the mission, the characters could drop some lines that are relative to the current story at the beginning or at some event point which would define their attitude
- Between missions (when they are finally safe !), maybe they could open up with some dialog system. That way, you can have long conversations with lots of text but only if the player chooses to.
- You could also have lots of collectibles that can be inspected, and their description would inform the player on the lore. Or even better, give secrets informations about the characters and unlock dialogs.
- When a character gets fatally wounded during a mission, you could integrate the dialog system at the end of the mission where you could exchange last words before the character dies, making it a strong moment for the player. He could also give you a final request (additional mission ?).
- And for the cutscenes, yeah I agree they should be very short and with as few texts as possible. But you could have many of them inside a single mission.