extemporize       
verb [ I ]  formal (UK usually extemporise) uk/ɪkˈstem.pər.aɪz/ us/ɪkˈstem.pɚ.aɪz/
 即席发言;即兴表演(或演奏)
to speak or perform without any preparation or thought
  I'd lost my notes and had to extemporize. 我把讲稿弄丢了,只好临场即兴发挥。 
 Synonyms: ad-libimprovise