My favourite example has to be Legends of Tomorrow, which in season 3 was dealing with a Big Bad that was a literal demon that spoke to people as a voice in their head.

This demon was voiced by John Noble, and after the main characters heard him for the first time, they identify that he sounds weirdly like the actor John Noble, time travel to New Zealand during the filming of Lord of the Rings, pop into Noble's trailer with a quick page for him to read for "Peter Jackson", so that The Atom could shrink down and fly into one of the demon's follower's ears, play the recording and trick them.

The entire sequence was nuts.