The term ‘red shirt’ came into being when people watching the original Star Trek series noticed that almost every single time security personnel, who wore red shirts, were sent down to a planet with Kirk and company that they were killed by the end of the mission. They were fodder for the robot/creature/woman/man that was out to hurt the more important members of the landing party. What mattered was that the officers of the crew were safe and that the ‘red shirts’ were expendable. Below is a montage of red shirt deaths from the original Star Trek series. I didn’t make it so don’t blame me for the music!
The term ‘red shirt’ is now used to represent any character in a film whose presence doesn’t bring much to the plot, but whose death somehow creates a sort of filler. This filler can be used to show the killer or something similar, but the ‘red shirt’ doesn’t matter as much as the fact that they die. As you can guess this is used quite frequently in horror films. If there is a haunted house filled with rowdy and horny teenager chances are a couple of them are going to sneak off, have sex and be killed in the process. They can be considered ‘red shirts’ because their roles aren’t meaningful and their deaths give the killer some more time on screen.
Is any of this fair to these poor expendable characters? Of course not! Can you imagine being an innocent character just going about your business when all of a sudden somebody jumps out and kills you? I mean, you only had five minutes of screen time and didn’t even get to show off your talent of being able to blow milk bubbles through your nose!