Game concept that's probably already been done: A videogame about possession, where you, the player, are the foreign entity possessing the character you are playing as.
The story's whole point is to get you out of the protagonist's head, and since you want to complete the game, it's presumable that you therefore want to help the protagonist get rid of you. While you control the playable character's body, their narration is their own, and frequently comments on what you're making them do, with remarks like "ugh, can we focus? we need to go to the thing" when you start twiddling with pointless things instead of progressing the plot.
And occasionally the character just refuses to do what you want them to do. Instead of an invisible wall barring you from going beyond the boundaries of the game, the character consciously halts and tells you "no, that's far enough. We can't just wander off." If you try to make the character attack NPCs that you can't fight, they shudder and shake their head, going "no, what's wrong with you? She's my friend!"
At first the game kind of railroads what you need to do next, which is specifically by the character you're controlling refusing to do what you're trying to make them do. Their whole goal is to get rid of you, get this evil damn thing out of their head. But gradually you can build up trust with the character, by leading them to things they want or need, and keeping them out of danger.
Consequently, allowing the character to die or get injured makes them harder to control, and more likely to refuse to do as you want. And the less they trust you, the harder the game is. While the sliding scale of trust might not be a visible mechanic in the game, you can hear it in the tone in which they talk to you. The voice lines are mostly the same, but when you start leading the character to a random direction, the same "where are you taking me?" can either be calm but intrigued, or distressed and afraid.
