on the spur of the moment vs suddenly

on the spur of the moment and suddenly both are adverbs.

Word	Noun	Adjective	Verb	Adverb
on the spur of the moment	No	No	No	Yes
suddenly	No	No	No	Yes

As adverbs, on the spur of the moment and suddenly are synonyms defined as:

on the spur of the moment and suddenly: quickly and without warning
on the spur of the moment and suddenly: happening unexpectedly
on the spur of the moment and suddenly: on impulse; without premeditation

Other synonyms of on the spur of the moment include abruptly, all of a sudden, dead, of a sudden, short.

on the spur of the moment (adverb)	suddenly (adverb)
quickly and without warning; happening unexpectedly; on impulse; without premeditation	quickly and without warning; happening unexpectedly; on impulse; without premeditation