extemporise vs improvize

extemporise and improvize both are verbs.

Word	Noun	Adjective	Verb	Adverb
extemporise	No	No	Yes	No
improvize	No	No	Yes	No

As verbs, extemporise and improvize are synonyms defined as:

Other synonyms of extemporise include ad-lib, extemporize, improvise.

extemporise (verb)	improvize (verb)
perform without preparation	perform without preparation