Is she more of a slave/submissive when she is (in bondage or not) simply taking it (whatever it is) or when she is actively serving?
I would say that the extent to which is a slave/submissive is dependent on her state of mind. A woman in inescapable bondage, being raped and tortured, may be a submissive slave, enduring this as a form of service, or she may be an unwilling victim, kidnapped, and fighting physically and psychologically to the utmost of her ability. And perhaps, with time and ruthless will, the unwilling victim may become a willing submissive and slave, as her will and her resistance are gradually broken down.
I would say that what makes me feel most dominant is bringing her to a new level of submission. It's about the journey as much as the destination. She is a work of art in progress. And progress can either be made when she is actively serving, or when things are done to her.
Dr Verloc