TV detectives have unusually good luck with photographs. When they're talking to a witness in the witness's home or office, out of all the photos available the detective will pick one out — usually a perfectly unremarkable one showing some person with his arm around the witness's shoulder — and ask, "Who's this?", and the answer will be relevant to the mystery at hand. Often it's not even a question, the detective is just admiring the photo.
"Oh yes, that's the victim's fiance."
What is very helpful is the way the witness will volunteer more information than any real person would. Instead of "That's my mother," they will answer, "That's my mother. This was taken just before she was torn apart by wolves."
Related to the Law of Conservation of Detail.