As examples were asked for, here are some:
You are trying to convince a mob of villagers to burn a witch. You hold a speech, possessing the local priest who is fairly good in speaking (ability score 5). Your demon has developed this ability a little himself, so your total score is 7. The GM decides that the villages actually would like to have a little burning, so he grants you +1 bonus dice.
You roll 8 dice, coming up 3 6 5 4 2 5 6 3. You keep the highest three: 6, 6 and 5. A very good result. You assign the default way, the two 6s to goal and effect and the 5 to cost. So you have a critical success in goal and effect and a normal success in cost.
As a result, you hold a speech for a while (the cost is in time) and you do great on both your primary goal (the witch gets burned enthusiastically) and your secondary goal (the burning leads to the villagers becoming more faithful, for example).
Now try the same with a random village guy in another village where they are not so aggressive. He has an ability score of 3, plus 2 for the demon, -1 because this is a peaceful village, for a total of 4 dice. You roll them and come up with 3 5 2 2. While you still convince them to burn the witch (you assign the 5 to goal), they are not very enthusiastic about it and will in fact hold it against you for the future (a 3 for effect, a failure). Alternatively, maybe they just burn her a little bit but before she dies someone rescues her and the rest of the village lets it happen. And the whole thing takes a really long time and some promises of free beer or something (cost 2, a failure).
Alternatively, the GM could decide that the cost is not in time, but in your own reputation. Maybe you will be the next one they suspect of witchcraft.