1. done, spoken, performed, etc., without special advance preparation; impromptu: an extemporaneous speech.
2. previously planned but delivered with the help of few or no notes: extemporaneous lectures.
3. speaking or performing with little or no advance preparation: extemporaneous actors.
4. made for the occasion, as a shelter.
While Winston Churchill is considered one of the greatest orators, he was anything but extemporaneous: without his notes and preparation before hand, his speech would fail.