Often they will embody a particular human stereotype, such as the barbaric warriors, scientific intellectuals, or priests and clerics.
I didn't believe that particular sexual stereotype, but I was betting that Olaf did.
You may believe he made that choice because he has assigned you to a particular religious stereotype in his own mind.
You've got the demographic data to back up that particular stereotype, have you?
Generally, though, the media and the police seem more actively involved in proselytising a particular stereotype.
For example, a person who happens to strongly match a particular stereotype is said to be: "Straight out of Central Casting."
If we think that a particular stereotype is unfair to the group it supposedly explains, then we're free to deliberately violate the stereotype.
It's just that particular stereotypes seem to partner 'mature'- such as grey hair, wrinkles and being a bit slow on the uptake.