extemporize /ɪkˈstɛmpəˌraɪz/ ( extemporizing, extemporized, extemporizes )

BRIT also extemporise

• 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]

  例：

  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。