While in general I agree with all that's been said, I also think it's worth mentioning that, in a sense, every well-written encounter with a character fleshes them out on some level, including the sex scenes. A girl chained in a dark basement might shiver, and remember past experiences, in flashback, giving a direct source of information, or she might chew her nails, fold her arms both in both petulance and defensiveness. A predator might consider his prey in ways that reveal his character, or the nature of the madness which drives him to his acts. A rapist might use colorful epithets, ethnic or regional slang, or speak with an accent that the author can describe in detail, or simply identify it-
"I'm going to fuck you, little bird," he told her, the words rolling almost pornographicly off his tongue in a Latin accent she lacked the experience to identify...
Now we know he's Latino, and well-spoken, and that she is unfamiliar with his culture- so he is not just a stranger, but likely a foreigner.
Likewise, you can include sultry, sensual elements in sections which are not actually sex scenes at all.
Have I mentioned I love words? Words are awesome.
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