Generally I wasn't too fond of The Office's sixth season, but it did contain my favorite episode, "Murder," in which Michael forces everyone to play a murder mystery game called Belles, Bourbon, and Bullets. Sadly, it isn't a real game, but that's okay since they give away the solution. But the best part of the episode is Dwight conducting his investigation. He says, "It's never the person you most suspect. It's also never the person you least suspect, since anyone with half a brain would suspect them the most. Therefore, I know the killer to be...the person I most medium suspect." It's funny, of course, but here's the thing--it's also true.
When it comes to mysteries, as a reader or viewer you certainly hope it isn't the most obvious suspect. There's no satisfaction in that being revealed. And the least suspicious person becomes the most suspected, because we've come to expect that. But a writer or director knows that, so there are two options: they either take the killer/kidnapper/whatever from the ranks of the "most medium suspects" or employ some twist or device that will seem to eliminate the "most/least suspects" before returning to them in the Reveal.
All I know is, from now on I'm calling that the Dwight Doctrine.