In most cases GMs let the players roll the opposed rolls. So the difficulty of your roll would be the skill of the enemy you are trying to affect, or is trying to affect you.
So if a PC is sneaking up on someone, he rolls Stealth. Difficulty is the Perception skill of his opponent. If he has Cun 2 and 1 rank in perception the difficulty of the roll is 1eD+1eC.
But if someone is sneaking up on the PC he rolls Perception. Difficulty is the Stealth skill of his opponent.









