A person that brushes up against your life but never effects your life in the slightest. Seat fillers are those people out there that seem to be there just to prove that you and your friends and family are not the only ones on the planet.
A seat filler in your life isn't even allowed to interact on a level deeper than standing in the shadows.
I know for a fact that they exist because I have personally tried to engage seat fillers in conversation or befriend them and it is impossible.
Seat fillers are a necessity. Otherwise it would be a very lonely planet.
At award ceremonies like the Oscars they have people who are paid to keep the seats full when the stars get up and move around or participate int eh show - that way the camera always see a full house.
The teller at my bank will not interact - if she is the perfect example is seat fillers she is an android of some sort - but I am not sure they have invented androids yet :)