Really, this is one of those situations that I've heard of so many times that it actually contributed to my impetus to start writing about all of this. I recently received a mail from a man in a long term relationship requesting advice. He indicates that while this relationship of his is healthy and solid, it's also exclusively vanilla, and he wishes to explore his dominant side. And further, that he feels his partner has submissive tendencies that would make adding elements of D/s to their sex life mutually enriching and exciting. And I feel that is so commonplace that I thought it might be useful to finally address it directly.
The truth is, if you are in a relationship and you identify strongly with a dominant or submissive nature, and you feel your partner has an unexpressed correspondingly opposite inclination, odds are you're completely right. If you do have this tendency, it would be difficult to imagine that it does not manifest in your behavior. And since you're in a relationship, it follows that this behavior is compatible with their preferences.
Introduction.
The first thing I'd say to those in this situation is that I don't really think the bedroom is the best venue to introduce more structured power exchange into an existing relationship. We tend to view sex as the culmination of a relationship; the consummation of a courtship. That courtship might be as brief as an evening in a nightclub or as long as long as months or years of dating. The point is that what happens before the sex sets the tone to be agreed upon by the physical intimacy that 'seals the deal' so to speak, even in casual contexts.
Since sex is largely by default considered sort of a later stage in a romantic relationship, and that stage is founded on the what a couple shares prior to it, trying to change the tone of a relationship with sex is often a bit like trying to alter the terms of a contract after it's signed. Often a partner can feel disturbed, tricked, alienated, or even cheated. Confronted by it, they can have an 'I didn't sign up for this' reaction.
So, my usual suggestion is to first try a more subtle sort of renegotiation of selective aspects of the relationship, slowly and easily, over a little bit of time where the partner can experience and accustom to it. In other words, try consciously demonstrating positive aspects of dominant behavior consistently in other parts of the relationship and allow them to respond. Hopefully, if we're correct in our suspicion of a submissive nature, that response should be a positive one.
Stated differently, a dominant can subtly try power exchange on for size for both parties, rather than engaging in some awkward discussion that abruptly risks alienation and rejection. The more open talk about power exchange can be postponed until after a positive example has been set for it, to show your partner what you have in mind, rather than resorting to some abstract hypothesis that might seem frightening.
In a sense, dominants have it easier than submissives in this situation. The reason being, a dominant essentially builds this social construct that others will encounter and inhabit to the degree they wish to be submissive to it. A dominant can build this structure on his own, so to speak, without requiring cooperation. But a submissive who builds such a construct is essentially being dominant, and not likely to draw a lot of personal emotional satisfaction from the arrangement. So submissives in this situation would need an entirely different approach that we can discuss another time.
Additionally, it has the benefit of giving the nascent dominant an opportunity to stick their 'toe in'. Often times in relationships there are things that we are lead to believe that we want, that we should want, that will make us happy and satisfied, but out self awareness and insight were mislead upon. And taking a little 'test run' might just alter their views of what they really do want in a relationship.