(1)An impromptu speech,adj,adv, is without preparation:-an impromptu speech makes you speak impromptu.
(2)What made you speechless?You are unable to speak esp because of deep feeling;speechless with surprise like this talk, Your anger left you speechless on stage, causes you unable to speak ,rage, making unnecasary fault or badly expressed
(3)The translation method from C/E is good.On stage you can communicate,converse, deliver an impromptu speech by translating out to audience and judging panels.Then express yourself,hold the conversation by saying something as told and talked.The topic to speak about is the comment on president Abean--vs--- president Obama(usa),discuss,mention,refer to them quick to relate some marks. Then to speak your mind, say what you think, speak honestly.speakout,state your opinion, voice your thoughts all in translation C/E method.Then the impromptu subject="You loses your way as a tourist ".eg:-The main points from usa---->-TW south:-eg:-round TW City South---bird's eyeview---oriental building is the Tower---Pagoda---Emperor---historical relics----scenic spots---->"Suddenly the weather changes"---meets a foreigne ----shows you the way because you're a stranger there---this is called "to take somebody round somewhere "as an impromptu.
(4)At the mean time buy the "Eng.Speech Book" by "Mr.Lau Yee":-who teaches you memorize 20 essays;deliver a one hour talk easily;express yourself freely by (1080=33x33 )vocab.applying translation method C/E pronouncingly, teaches you how to discuss,converse,+deliver a speech.Also listening to radio news +Bill-Board charts for pop-music=speaking !!!This answer your problem" How to get ready for Oral presentation+speak "informally !!!Referring to other orators, spokespersons, responses please !.With best of wishes !!!