Detachment. It requires the discipline to control your emotions to such a degree that they are not physically or mentally filtering a situation except in that a way you intend. That is what allows you to see a larger and more accurate picture than others which can create a strategic advantage. Simply repressing emotion is only one part of the skill. Being able to elicit an emotion within yourself that might be quite contrary to your real emotion (and making it convincing) is very difficult. For most people, emotions are like a stream that simply just flows and they go where is carries them. They don’t really think about it much and therein is the key. The social engineer learns to install dams, flood gates and aqua ducts in their own stream so that they can control their emotions as required to alter the path of others’ streams to serve the end goal.
The obvious example that most engineers think of is managing the fear of getting caught, but a more common problem is managing the emotion that comes right before the seizing of the prize when your success is seemingly imminent. This kind of emotional leakage triggers warning lights in others, even though it is not a negative emotion that is leaking.