Speaking parts ruin television
You're watching your hour-long drama/mystery/police procedural show on television, and your heroes engage in conversation with a hitherto unknown guy or gal on the street, who just happens to be hanging around. Given that speaking parts pay much more than stand-around-and-nod-silently parts, you can bet your sweet plotline that the guy or gal had something to do with the drama/mystery/dead guy. It's a dead giveaway, but what are you going to do? Spend more to have a lot of red herrings jabbering away? I don't know. Do you have any suggestions?
Sometimes it's even more subtle, like the camera lingering just a bit too long on some bystander who's making a telltale face of some kind. I don't know what the answer is, but in the case of my movie gripe in my comment to your Oscars post, I think the answer is clearly to use an unknown actor instead of someone recognizable.
The thing with the drama/mystery/police procedural show is that the formula is so familiar. It's almost like a competition at our house now: Who'll be first to figure out the entire plot? Sometimes we nail it after seeing only the opening vignette.
Lately for us it has been the unexpected appearance of Bruce Davison. But just after the opening vignette, can you really be sure? Oh, you can sound sure, for the more effective unsettling of your opponent, but can you be sure?