wow... really good question Sher. My current SO started as a friend and sometimes lover, whom i shared some of my tastes with but not the really extreme. The first time was very scary. A lot of crying was involved, to be honest - it was so emotional, fearing that she'd think me a freak. It started out as a weekend where i shared a side of me i don't share with many, spurred mostly by my involvment on E, funnily enough, letting her dominate me entirely, spurring her on to a place that we'd never gone before.
I have a journal, well, several of them, really, that relates only to sex. fantasies, solo experiments, stories. We sat down later that night and i let her read thru them while i was there to explain and answer questions and just talk about everything. It was terrifying, to be honest, and yet, we'd already laid a foundation of trust down to where i could do it. Fortunately, at the time, we'd already played in some light bondage with her the dominant partner and spankings and such, so it wasn't completely out of the blue... We still keep the communications open about it, too. We talk a lot about things we do, want to do, don't feel comfortable doing, what makes us wet and what doesn't. If you're not talking about it, you're doing something wrong and it's not going to work, in my opinion.
Something else that has come up from time to time in our relationship, by the way, is that while it was easy to talk about it and for her to accept some of my... well, needs, desires, kinks, what have yous, the reality is often a little harder to deal with and it's has lead to slowing down, stopping, reversing, etc, more then once when something comes out that she is not quite prepared to deal with.
i hope this is coherant enough as an answer? *smiles* i really don't want to go into ALL the details...