Well, it's a fun alternative to playing, a great way to get involved and a brilliant way to enhance the experience of the players.
That's easy, just play a role. Depending on the type of event you could be a civilian, a intel operative or even a spy or saboteur!
That's the best bit, nothing just your time actors are strictly volunteer positions.
Actors on events will have a role to play within the event. Most commonly it will be as civ-pop (Civilian Population). You primary purpose will be to interact with the players to make their experience more realistic.
Anyone who attends as an actor will need to understand fully the following points:
You will never be subjected to abuse, mental or physical.