Well, this is about it.
They watch various video monitors, the actual amount depending on the size and surveillance budget of particular casinos, and very seldom see anything actually happen. Their job is pretty boring to say the least, and many operators actually do lose consciousness up there while you`re gambling or cheating down there in the casino, depending of course, on who you are.
Casinos tend to depend too heavily on their cameras, not realizing that these state-of-the-art cameras cannot tap the surveillance operators on the shoulder and say, "Hey, Rip Van Winkle, I just filmed a cheat move go down on blackjack table number 13!".