【Spontaneous】 ,  【impulsive】 ,  【instinctive】 ,  【automatic】 ,  【mechanical】  in application to persons or their movements, acts, and utterances mean acting or activated without apparent thought or deliberation.

【Spontaneous】 can describe whatever is not affected or effected by an external or internal compulsion of the will and comes about so naturally that it seems unpremeditated as well as unprompted.

【Impulsive】 applies to someone or something actuated suddenly and impetuously under the stress of the feeling or spirit of the moment and seeming to be involuntary and forced by emotion rather than voluntary and natural.

【Instinctive】 implies the guiding influence of instinct and a native and unreasoned prompting to actions characteristic of the species and presumably contributing to its life and well-being; when referred to human beings, the term is applied to actions, movements, or feelings which are instantaneous, unwilled, and often unconscious (as reflex movements, habitual actions, or specific responses to stimuli).

【Automatic】  and  【mechanical】  apply to what at least to outward appearances seems to engage neither the mind nor the emotions and to suggest the operation of a machine.

But  【automatic】 , like  【instinctive】 , stresses promptness in the response. It differs from  【instinctive】 , however, in implying adaptability to changing circumstances and readiness to react or to respond immediately and unvaryingly each time a given situation or stimulus recurs.

【Mechanical】 , on the other hand, stresses the lifeless and, often, the perfunctory character of the response. It does not, as  【automatic】 often does, suggest perfect discipline; rather, it suggests a mind dulled by repetition of the act, motion, or operation and capable only of routine performance.