In Black & White, an actor is any NPC that is associated with a gold, silver or non-scroll challenge. If the player is to hit the S key to display status about villagers, they will notice that actors will be tagged as "Actor" and have their description in white letters, indicating that they don't belong to any village. Some actors will have a unique character model (e.g. Sable or the sculptor), making them different from other villagers.
Some actors will join the nearest player-controlled village upon completion of their challenge, doing tasks like any other villager. This will cause their "actor" tag to be removed. Actors who do not or cannot join with a nearby village will have their tag replaced with the "Vagrant Start" tag, which will make them wander aimlessly about the island until they die.
Most actors are 18 years old when they spawn.