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Source: https://writers.stackexchange.com/q/15850
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

I'm in need of an idea to advance my fictional crime plot where the protagonists of a larger (~30) group realize that the murderer (of a single murder) must be one of them, someone hiding in the group, playing innocent with everyone else. I am looking mostly for inspiration, short stories, novels that are built along similar lines, so that I can extract and bend the story elements according to my needs. To make it clear: I'm not only looking for direct ideas, but also for similar **sources**. So you might find it more of an `identify-this-plot`-type question than a purely opinion-based one.

The closest motif that is very similar is Asimov's short story "[Little lost robot](http://en.wikipedia.org/wiki/Little_Lost_Robot)" from "I, robot" (not the movie). In this sci-fi novel, a robot gets the offhand instruction to "get lost", and he takes it seriously: he tries to merge within a larger population of 62 similar robots lying to any direct questioning about it's origin and intentions (thereby effectively defying the 2nd Law of Robotics: obeying humans --\> a free-roaming, lying robot poses a serious threat). The protagonist robot psychologist (Susan Calvin), after hours of futile questioning, realizes that out of the 63 robots (62 normal, 1 lying), only one is prepared with a specific piece of knowlege (the one lying), but he himself does not know that the other 62 is lacking this knowlege. Calvin cleverly uses this discrepancy to force the lier to reveal himself by externalizing the internal difference to appear as a clearly visible difference in behaviour.

So abstracting away: I need ideas where the protagonists realize that

- there is a nontrivial **differentiating piece of information** available for them (and for the Reader), which ensures that the otherwise undifferentiable murderer differs from the innocents, AND
- this **information must be hidden** i.e. cannot be trivially available to reveal the murderer, AND
- the **murderer does not know** this difference exists (thus the trivial solution "Are you the murderer?" wouldn't do any good), AND
- the murderer can be trapped by exploiting the differentiating information in a way that if the murderer would react to it, no matter hows casually he would do it, would reveal the difference at an obvious level for the audience and for the Reader.

That is, during the confrontation, the murderer still reacts to the trap question according to his best intention to hide himself, assuming that he HAS to know this knowledge (as he believes all the other members of the group is in possession of this knowledge). Since the protagonists know, that no other member of the larger group should know about this information, the murderer is revealed, against his intention.

Original score: 0