Being able to judge when to best give feedback/comments certainly helps. A few words after a maneuver or before trying again can be more effective than 5 minutes talk much later when the trainee doesn't even remember what specific maneuver you're talking about. Silence at the right time certainly helps but especially in simulator training, where demonstrating something yourself is not really an option, talking a trainee through a maneuver may be required as well and requires careful judgement to avoid "overfilling and spillage".
The ability to remain calm and not take even rather stupid trainee actions personally certainly helps as well. How important that is depends mostly on the trainee. Even feedback delivered angrily can be to the point, there's just a higher risk it arouses so much emotion that the point doesn't come through.
My experience is mainly in airline training, everything else was long ago.