extemporize

1. V-I If you extemporize, you speak, act, or perform something immediately, without rehearsing or preparing it beforehand. 即席演说; 当场表演[正式]

He completely departed from the text and extemporized in a very energetic fashion. 他完全脱了稿并以一种精力非常充沛的样子即席演说。