Learn English – Synonyms for “palette”

A voluntary action is one that the actor, employing free will, chooses to take.

An involuntary action is one that takes place out of the control of the actor, or against the actor's will.

Spontaneous has several related definitions. A spontaneous action can be one that the actor takes without a significant amount of forethought ("a spontaneous purchase"). In that case, the spontaneous action is voluntary.

On the other hand, spontaneous can also refer to something that occurs with no apparent cause ("spontaneous combustion"). By its nature, this activity rarely involves an overt "actor" per se. When it involves a living organism ("spontaneous growth", "spontaneous motion", etc.), the action is probably involuntary.

Spontaneous has a few other definitions as well. Generally, you'll have to discern the difference from context. When you see the word "spontaneous" used to describe an action taken by a clearly definable actor, though, the action is probably voluntary (although it might not be).

I think if used in the context you've described curious will consistently have a positive connotation; I can't conceive of how curiosity about an academic subject could be taken negatively.

Perhaps the word 'interested' is less likely to be construed negatively. "She was interested in biology".

I think that any negative connotation attached to any word that describes inquisitiveness stems from the human desire for privacy; there will always be certain subjects that people don't want examined, as such there will always be the potential for any type of inquisitiveness to be spun negatively.