Actors can be primary or secondary actors.  Primary actors initiate a use case, while secondary actors support a use case or receive something of value from the use case.  While this answer might score you some points in the interview, there is another way to classify actors that is important to know and can show that you understand some of the finer points of use case diagramming.

Actors can be:

1. Human
2. Systems/Software
3. Hardware
4. Timer/Clock

Many analysts miss key actors because during the use case diagramming process because they fall into the pitfall of only identifying human actors.  Categorizing use case actors in this ways helps the analyst ensure they haven’t overlooked any critical actors within the use case diagram.