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