WordNet sense 1 (proceeding from natural feeling or impulse without external stimulus):

unprompted

WordNet sense 2 (happening or arising without apparent external cause):

spontaneous, self-generated

WordNet sense 3 (not premeditated):

unpremeditated

WordNet sense 4 (having the power of driving or impelling):

driving

WordNet sense 5 (characterized by action or forcefulness or force of personality):

dynamic, dynamical

WordNet sense 6 (determined by chance or impulse or whim rather than by necessity or reason):

capricious, whimsical

WordNet sense 7 (based on or subject to individual discretion or preference or sometimes impulse or caprice):

arbitrary

WordNet sense 8 (characterized by undue haste and lack of thought or deliberation):

hotheaded, impetuous, madcap, brainish, tearaway