I have great admiration for those who can think on their feet and can improvise a massive speech or a stand-up routine or something similar. For me, I need at least a little bit of rehearsing before I can even do anything in front of a crowd of people. Even if they're my friends or people who I know won't judge me, I still feel the need to have a modicum of preparation before doing any public speaking. I just need the security of having something to work from.