extemporary
The talk was presented extemporarily by one of the team members.
She wanted to speak extemporarily on one of the points that had been discussed.
At the last minute, I jotted down a few ideas that I wanted to include extemporarily in my lecture.
He always preached extemporarily to his congregation.
It's easy to tell whether someone is speaking extemporarily, or whether they have learned their speech by heart.

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