suddenly vs on the spur of the moment

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

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

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

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

Other synonyms of suddenly include abruptly, all of a sudden, dead, of a sudden, short.

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