impromptu

a. 即席的,即兴的
ad. 即兴的(地),无准备的(地)

It usually take me more than three weeks to prepare a good impromptu speech.

为了准备一则即席的好讲辞，我通常要三个星期以上。

An impromptu visit broke up the long afternoon.

突然的造访打破了漫长的午后时光