Speaking of my job being "invisible"...here's a good example.
Last game session, the PCs found a necklace with a symbol on it that matched the clan's symbol. It's extremely old. When they get back to their village, nobody will know what it is...all will wonder at it. Is it just a silver necklace made generations ago by an ancestor clansman? Or, is it something more?
Magic in Conan's universe is low, but it exists. I keep sorcerous items scarce, and like this necklace, many times, a player won't know what he's got. It could very well be just a silver necklace. Or....it could be something...more.
In a way, it's a "hook" I've put into the game.
Now...behind the GM's curtain...
I placed this in the game without knowing what I would do with it. I do this a lot. If I never come up with an idea, it becomes a cool roleplaying item--a necklace with the clan symbol froma warrior that lived many generations ago--and that's it.
But, I've left myself an "out" with it. I can add a power to it anytime I see fit, and from the players' perspective, they'll think that the necklace was put into the game for whatever reason related to the power I give it.
Today, right now, the necklace is specifically nothing, magically speaking. But, had the demon and his undead starting getting the better of the two barely-alive PCs (see my post above), THEN, one of my contingency plans would have been to give the necklace power.
"Just as the undead move in....as you stand back to back with your clansman and brother, weapons at the ready...the sky parts with a bright beam of light. Although it's hours past midnight, the heavens open as if the sun has come out prematurely. A quick look tells you that it is not the sun, but the moon that sheds this light.
"Is your eye playing tricks on you? The moon is full, bright in the sky. The overcast clouds have parted to allow it to shine uniquely on you, in a circumfrence around you. In the shadows on the moon....the shapes....is that the symbol of your clan you see on the moon?
"The walking dead repel from this light. The demon throws its arms up to cover its eyes.
"In a moment, they're gone. The undead and the demon.
"And so is the light from the moon.
"The thunderstorm continues to drench you and your brother. Lightning flashes across the sky.
"What do you want to do?"
You see...this would have seemed like GM intervention had I not put the necklace in the game in a previous game session.
The player just think that the circumstances somehow activated the sorcerery in the necklace.
For real...I don't really know what I'm going to do with this necklace. It may have a completely different power the next time I need it for a contingency plan. And, the possiblty is still strong that it will never become anything in the game but an old, cool, ancient silver necklace.
The longer the necklace is in the game, the less likely it will seem to the players that the GM is saving their butts in a rather deadly game (that is not near as deadly as the Players perceive).
Even if I had done what I said above, with the moon and what not, I've still got a pretty mysterious magic item on my hands--in a low magic game world, where magic, when it does exist, is usually pretty powerful.
So, I can take what I did and then make some rules for the necklace, slowly allowing the Players to figure out how the thing works.
It's just a GMing technique.