What makes some people look down a steep and slippery mountain and say to themselves, "Weee heee, I can get down this slope REALLY FAST!" and yet fill other people with absolute terror?
What makes some people approach a public speaking event, standing on a stage in front of 900 people, with excitement and anticipation, and give other people the cold sweats several months in advance of the event?
Part could be fear of the unknown (although for my son, he approaches every ski-able mountain with the same glee, whether he has been down it already or not). Certainly some certainty about what is around the corner, or down the hill, or some familiarity with my audience, is helpful. You can check out the plan, get some local knowledge (who's been down that hill or worked with that group before?), do what you need to to inform yourself about what is coming.
Part could be confidence in your ability to handle new and unexpected things. This could come from a great deal of practice, so taking hours of lessons, clocking hours of snow time, and getting back up over and over again can help (if you don't hit a tree and break your arm one of those times, which can set you back both physically and mentally, let me assure you).
This also works with public speaking and facilitation - after I have done a run of workshops, I feel like I start from a position of confidence in front of a group. And even after a dud workshop session or presentation for whatever reason (and we have all had them -my first Toastmasters Table topic was a real blooper), you need to reflect on that and try to remember more of what you learned next time (lean forward, dig in those edges, prepare yourself, keep cool). Learning from more experienced speakers and facilitators (as well as skiers) is a great way to learn - be it in lessons or from mentoring/shadowing/keen observation.
Part could be sheer bravado, but I am not sure how I can map that over to public speaking or facilitation - except that if you believe it enough there might be some self-fulfilling prophecy there. This might relate to just that instinctive feeling that you can do something; that you have the right tools and equipment and muscles, and master them, you have good general awareness, and feel that normally when you try something and give it your best, it works out. (This could be a pre-tree collision feeling, but can come back with some additional effort and if you don't give up, I am assured.)
All I can say at this point, is that I look at an audience at a workshop, or in an auditorium with much less trepidation than I do that mountain slope and I'm doing my best to apply my learning from one to the other!