Watch the Footage archive

So I remember that at one point I had an insane WMG about a connecting theme between the detectives' cases, though I can't even remember what that was anymore (which is probably a good thing).
But was there a connecting theme? (and if so, what?)
What was the basis on which the cases were chosen? (other than "this can make a convincing narrative", I guess...)

Edit: I'm talking about OTHER than the cases that relate to the other characters, of course.