Everyone else plays a suspect: their goal in character is to make the detective declare that they are the murderer. Who actually killed the victim isn't defined until end of play: each person comes up with a motive for their character, and will have the ability to plant clues throughout the game (a limited number, perhaps, or some sort of clue economy where the authority to introduce new clues moves around from player to player).
But in character, the suspects want to avoid being caught. So they can't ever state that they are the murderer, and they need to avoid directly revealing their motive or how they could have done it. (Maybe the clue economy can be used here to avoid people acting too overtly like the murderer.)
After some amount of time, the detective decides that he has enough evidence to solve the case, fingers someone, and tries to explain away any conflicting evidence.
Mainly, I'm thinking about the tension between suspect player and suspect character, and how that tension could be used to produce satisfying gameplay. And if this could be used to create a near zero prep LARP or tabletop game. (For a LARP especially, suspects should have their own interrelations and the clue economy should encourage them to interact away from the detective.) But would this work? Would it crash and burn in some obvious way I'm missing? Any way to make it better?