I was taught by at least two speech professors that an impromptu speech is one that is given on the spot, with no formal preparation, whereas an extempore speech is one that is prepared in advance. If this is so--contrary to some dictionary definitions, could the similarity between the words "extemporaneous" and "spontaneous" have created confusion in this regard (as they both contain the ending letters -aneous)?
An impromptu speech is, after all, given spontaneously. Could an extempore/extemporaneous speech be neither strictly spontaneous nor strictly planned but an admixture of both?
(Perhaps I've answered my own question by suggesting an extempore speech has elements of both spontaneity and planning. Having done a good deal of public speaking, I have on many occasions departed from my prepared remarks by either adding or subtracting elements in an impromptu or spontaneous way. A sudden "brainstorm" could trigger such redacting, or even a puzzled look on the faces of my auditors(!), not to mention time constraints.)