extempore
These extemporal verses were written after a visit to the coast.
He was a performer with a pleasant, extemporal wit.
She has a gift for extemporal comedy.
In those days, extemporal acting was always part of performing a play.
He believed his extemporal and spontaneous style of writing to be the ideal.

SMART Vocabulary: related words and phrases

Not expected or planned
abrupt end
abrupt halt
abruptly
abruptness
ad hoc
inadvertency
inadvertent
inadvertently
inconceivable
inconceivably
sine die
spirit
spontaneity
spontaneous
spontaneously
unsuspected
untimely
untoward
unwilled
unwitting