People or persons who have no meaning in this world, they have a boring personality, and they are rather dull. People who are outgoing are usually not fill-ins.
Michael jackson is the perfect example of someone who is not a fill-in.
Refers to an act of grafitti art. To use two or more spraycan colors to fill-in an outline of text on public property.
Did you end up going out to graff last night?
Yea, I did a fill-in under the bridge, you should check it out.