Not rehearsed. See Synonyms at extemporaneous.(adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "unrehearsed" in a sentence
"That the evacuation was unplanned and unrehearsed is remarkable."
"You want to take the interviewer off script to net some unrehearsed answers."
"So, having listened to this unrehearsed, unperformed play, the writer David Seidler was in the audience, and I got my nice, bold English husband to go up and ask him if he would let Tom see the script, and he said, 'Yes.'"