We humans are having habit of speaking to each other and rely on this mode of communication to the maximum. Speaking without thoughts is a gossip whereas speaking with thoughts, direction, on theme is a lecture. But speaking in stipulated time with topic and on the spur of moment to a niche audience is extempore speech.
Extempore Speech must come from within while maintaining flow of thoughts focused on topic. It has to be natural so that it must not look copy or repeat. You need to enjoy speaking as much as your audience.
With natural instinct, body language or non verbal communication will supplement verbal skills and you would perform an orchestra of speaking with high chances of enthralling audience.