So, I'm working on a video game wherein there's a lot of supernatural stuff "legitimately" going on around the player, but the player character is also schizophrenic, and realizes as much. So what's really happening, and how much the player is actually contributing to solving the problems presented in the plot will always be questionable.
The player's schizophrenia manifests as voices and hallucinations. Some characters in the story will be revealed as fake, and others won't. The game takes place in a near-future war-torn world, riddled with dangerous unexploded ordinances, automatic turrets, maybe even some robots (maybe), and such.
At one part in the story, the player will be traveling cross-country with 3 or more comrades who do not realize that the player has this problem. The player character can't tell them, because then the player would be a liability on the group and would be cast out. One of the comrades is a hallucination, and the player has to find out which one.
For this part of the plot, I plan to put pressure on the player to find out which friend is a hallucination, using situations where the player has to depend on his friends. For example: the player and two friends are separated from the rest of the group; they find themselves needing to cross a mine field, and the two friends take separate paths through the field, both urging the player to follow them for safety (I might not use that idea; it's just an example).
I've got ideas for how to give the player clues as to which friend is a hallucination after the player already knows that one of them is, but I don't know how to initially make it clear that one of them is definitely fake without telling which one.
What do you guys think? How can I reveal to the player that one of the NPCs in his party is a hallucination without revealing which one?
ETA: I'd like it if your answers didn't take into consideration any mechanical gameplay limitations necessarily. Feel free to also consider the question as if I told you I was writing a book and wanted to reveal something to the main character. I'll use what parts of your answers I can use in order to work out my final product.