extemporize

1. V-I If you extemporize, you speak, act, or perform something immediately, without rehearsing or preparing it beforehand. 即席演說; 當(dāng)場表演[正式]

He completely departed from the text and extemporized in a very energetic fashion. 他完全脫了稿并以一種精力非常充沛的樣子即席演說。