Their is no answer for any of your questions because everything depends on everything. In order to approach people you just have to do it. I believe someone who puts it best (at least I relate to it) is that Calen Morelli said in a youtube video that he has around 5 seconds to just walk up to a stranger and perform. If the approach doesn't happen in those 5 seconds he won't do it because of the anxiety. You gotta just do it no matter how you feel. Whats the worst that could happen? Thy say no thanks? Doesn't seem bad to me. Next, the best way to get people interested if we are talking about walk around is to do something quick and visual. With this you don't even need to say "Hey watch this" you just do it sometimes and they will notice if you are in their line of sight. It is really hard nowadays with everyone always on their stupid cellphone. Do not start out with a card trick ever unless you are making cards disappear with a back palm. Thats one of the only things that works as an opener. Card tricks do not work as walk around openers because they are not visual and they involve the spectator. You can't involve the spectator on your first effect because you are assuming they want to watch. Lastly, being courageous and confident comes with actually doing it like in my first point. You WILL GAIN CONFIDENCE when you just keep approaching and actually performing to everyone, no matter how you feel. I know how hard it is to approach and perform for strangers but you gotta fight the bad feeling you get in your body and push past it. Become a new person and grow with the experience. You will become confident.