A few years ago, after watching one too many whodunnit TV mysterys, I coined my

Law of Economy of Characters

The killer is in­nocu­ous­ly introduced in the first 20 minutes.

In real life, the killer may not be known until late in the in­ves­ti­ga­tion—if ever.

In a TV mystery, any non-recurring character who gets more than a few lines has to be a potential suspect—to the audience. The character is not there gra­tu­itous­ly. Their salary is being paid for a reason.

It’s not uni­ver­sal­ly true, but it works more often than not. It’s less true in books, where throwaway characters are easy to introduce.

Googling around, I found the following, attributed to Roger Ebert:

Ebert’s Law of Con­ser­va­tion of Characters:

Any main character whose purpose is not readily apparent must be more important than he or she seems

Ebert’s 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. Cf. the friendly neighbor in Lady In White. (See also Un­mo­ti­vat­ed Closeup)

I’m in good company.