In order to effect somebody with a spell you must have a line of sight on them. But what if it's a sustained spell and you succed in casting it then they move out of your los? For instance can you sustain that armor spell on someone even when they're not where you can see them? What about control thoughts/actions?

You don't need LOS to sustain a spell.

You need LOS to sustain counterspelling .

Counterspelling isn't sustained. It just is.

It doesn't either way. But if LOS is required then Invisibility would be kind of pointless.

That, and spell descriptions that state things like "if the mage isn't present (it's easier to resist the spell, etc)", show quite clearly that a spell can be sustained without needing to see the target.

It never says that sustaining requires LOS. As soon as casting is done, LOS is no longer needed.

But you´ll never know how much the target is affected.

Which can get you into interesting situations where, say, a runner's teammate is dead, and the cops are following the astral trail from the spell being sustained on his body, without the mage knowing about it. Hilarity ensues...

This works best with a high force Improved Invisibility spell and a busy street. Invisible guy croses the street to get to his target and none of the cars can see him, of course. The magician continues sustaining the spell and they police only find his body because they eventually trip over it.

Just remember to clean your astral signature after, Hyz! No one likes a sloppy assassin. Well, except for the cops.