I think this is done primarily when the character in question has been introduced before, but it was far enough in the past that the audience may not remember.
In Episodic Series Television, we usually see such characters in the "Previously on Long Story": vignettes (2-3 seconds) at the beginning of a show, to remind the audience of characters, events or something said by a character from previous episodes.
In a comic book either approach could be used; reproduce a panel with a key line of dialogue but with a caption like "One year ago in the Balkan adventure:".
Sometimes, directors just want more than a name for realism, so they do a full makeup of somebody just for a short scene. In Star Trek The Next Generation, this is often done with officers from Star Fleet Headquarters. If they wanted to do it cheap, they could have sent an email or voice mail; but for the drama Captain Picard meets them in person in a video conference, the actor (in costume) may get less than 3 seconds on screen.