To do or perform (something) without prior preparation or practice: extemporized an acceptance speech.
intransitive verb
To perform an act or utter something in an impromptu manner; improvise: “bravely demonstrating his ability to extemporize intelligently” (William Safire)
derivatives
ex·teḿpo·ri·zátion
noun
ex·teḿpo·riźer
noun
The American Heritage® Dictionary of the English Language, Fourth Edition