An Actor is a feature that let's you create an person (NPC) from various of skins
Adding an Actor (Actors -> Add Actor)
You must first first select the actor model using the "Y" and "N" key. After thet you have to select what weapons the actor will use and it's animation (see here for a list of animations). You will then be bought to another menu, here you can select the health for your actor (100 (normal), 250, 500, 1000, 2500). You will then have to select the alliance of the actor. If it is an ally then it is on your side. If it is an enemy then it is against you. If you select "Enemy" then you will have to select it's behaviour (Normal, Hold Position, Attack Direct, Hold and Direct). If you select it as an ally then you also have to set behaviour; (Normal, Hold Position, Follow Player, Must Survive, Hold Position and Must Survive, Follow Player and Must Survive). If an actor that "Must Survive" is killed the mission will fail.You then must select it's weapon accuracy (10 (poor) 25 (decent) 50 (good) 75 (very good) 100 (perfect) ), weather or not it can be killed by a headshot (off means it cannot be killed by a headshot, on means it can). The actor will then be spawned after the currently selected objective.