The mayor of New York made an impromptu appearance at the trade union conference. Or u can say .. He gave an impromptu speech when asked to say a few words.
I delivered an impromptu speech in front of a very big crowd.
The mayor of New York made an impromptu appearance at the trade union conference.
without previous preparation impromptu speech was made to the crowd
He surprised the cast by making an impromptu appearance on stage.