extemporal

Keats had in no small degree the ‘fine extemporal vein’ with ‘invention quicker than his eye.’
The Bibliotaph|Leon H. Vincent
Sir Nathaniel, will you hear an extemporal epitaph on the death of the deer?
Shakespeare's Lost Years in London, 1586-1592|Arthur Acheson
Men of great genius had a passion for performing in these extemporal comedies.
Curiosities of Literature, Vol. II (of 3)|Isaac Disraeli
But there were probably some secret aids in this singular art of Extemporal Comedy which the pride of the artist has concealed.
Curiosities of Literature, Vol. II (of 3)|Isaac Disraeli

Stowe records of one of our actors that "he had a quick, delicate, refined, extemporal wit."
Curiosities of Literature, Vol. II (of 3)|Isaac Disraeli