extemporarily
He's a master of the art of extemporary public speaking.
I can't give you an extemporary answer to your question.
He prefers to pray in an extemporary way.
I enjoyed listening to the child's extemporary singing.

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