Basically it was a running joke/conversation about how the passenger was just a figment of the driver’s imagination. And how anyone in the back seat was a figment of the figment of the driver’s imagination imagination… (confused yet? good that was the whole point.) It would switch off depending on who was driving.
We’d get into some really bizarre conversations that way. It didn’t help that most of these were at 3:00 or 4:00 in the morning on the way home from some place. I guess we used it mainly as a way to keep each other awake, so the driver wouldn’t fall asleep at the wheel and drive into a ditch, or off a bridge, or something.