Can the "Law of Economy of Characters" be circumvented?

[Skott]

@Roger Ebert

Law of Economy of Characters

Movie budgets make it impossible for any film to contain unnecessary characters. Therefore, all characters in a movie are necessary to the story—even those who do not seem to be. Sophisticated viewers can use this Law to deduce the identity of a person being kept secret by the movie’s plot: This “mystery” person is always the only character in the movie who seems otherwise extraneous.

I’ve used this law on more than one occasion, especially when watching any of the C.S.I. incarnations. The thing that really brought it to mind was after reading The Bone Collector (based on a recommendation in a request for puzzle fiction): At the very beginning, they mention the serial killer case that incapcitated Rhymes turned out to be committed by a cop, and I was thinking, “They’re not foreshadowing that overused cliche, are they? That one of the minor cop characters, who’d be familar with Rhyme’s methods, would be the one who did it?” However,

…it was only used as a distraction and the actual killer turned out to be Rhyme’s doctor who had apparently read up on spinal injuries for a few weeks while in an insane asylum.

Which was a great disappointment to me, because it was almost like the author said to himself, “Well, who’s the most unlikely current character I can make the killer regardless of logic?” The answer, to me, was completely unbelievable.

So my question is, whether in film, TV, or literature, can this law be circumvented? Or will the mysterious perpetrator always be someone we’ve already met in the story? And… which is better?

[Ludovic]

There are plenty of extraneous characters in Coen Brother’s movies, which is one of the things that makes them so life-like, or should I say, non-movie-like. (Another is palpable embarassment, tension, and fumbling during conversation, which happens a lot more in real life than in the movies.)