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

I'm in good company.