That’s who we are talking about. Sometimes you need to write a scene that expands or explains what is going on but doesn’t require the presence of characters higher up the character echelon. It could be the victim of a murder, moments before he dies, or a late night guard watching the cameras as the MC tries to sneak into the building. The temptation here is to fill the narrative void with a flat sterotype character who says his lines, does his moves and disappears without ever being seen again. But if the scene is important, then the character presence is important and should be treated accordingly. The trick is too add just enough detail to make the reader care about the character on the page without turning it a character study (although that is a nice option if you can work it in).
So, when you need a character in a hurry, call on the one-scene-wonder!