Leave Your Misconceptions At The Door
We might as well get this out of the way first: A lot of people feel guilt (beyond the hot “ooh, this is so naughty” stuff) about roleplay. Often, the concern is that playing pretend with your sex partner might suggest that you don't find them attractive as themselves. Sometimes, especially in fantasies involving force or naiveté, guilt comes from the worry that pretending you're doing "immoral" or illegal things is only a step away from actually doing them.
Fantasy is a normal and healthy union between our sex drives and our big, creative brains — it's sheer biology. It's how we're wired. Some partners will be cool with this and excited to play; others won't (and that's okay). If you've ever fantasized about choking your boss or felt your blood pumping as you killed a “bad guy” who was actually just a collection of code and pixels on a computer screen, you know that fantasy and reality are more than a little removed from one another. When that barrier breaks down, it's because of other problems — not because of fantasies themselves.
Now that you feel secure in the happy normalcy of whatever your roleplaying desire may be, let's talk about how to approach non-traditional scenarios. A lot of this will depend, as do so many things, on how well you already communicate with your partner. You may want to have a general conversation about roleplay before addressing your particular ideas. Talking about the concept in general gives you the opportunity to reassure your partner if they express any concerns, and your partner is likely to be more open and feel less threatened during a theoretical discussion than one that's goal-oriented. In other words, "Hey, what do you think about people who roleplay for fun sometimes in bed?" may get you a less defensive or panicky response than "I was thinking it would be great if you put me on a leash and I pretended I was a dog tonight. What do you say?"
As with any new intimate activity, negotiation and boundary setting is key. Maybe your partner is keen to help you act out the fantasy of being a rock star banging a fan backstage, but they want to make sure it doesn't go down the “what a stupid little groupie” road. Or perhaps you're anxious to try fantasizing about being a virgin, but the usual “Ow, it hurts, you're so big!” trope doesn't appeal. Don't just assume your partner knows what you mean by hot Klingon nookie — talk it over! Believe me, losing the element of surprise is well worth having a roleplay scene, with comfortable partners, that's unlikely to go off the rails. There's always the opportunity to ad-lib during repeat performances, when you both feel secure in your understanding of the roleplay's scope.
So you've negotiated the scene, made a date, and the appointed time is fast approaching...only you're not sure how you're going to imagine you're a posh and pampered English king when your date's coming over to your ratty studio apartment. What to do?
It may sound hokey, but close your eyes. Let yourself sink into the fantasy. Take note of everything that appeals to your senses in it: What do you imagine seeing? Hearing? Touching? Tasting? Smelling? Nearly any roleplay can be improved upon if you play with your senses. Maybe your saggy double bed starts feeling kingly if you spend a few bucks to trick it out with a preposterous number of pillows. If the casual gluttony of kings past turns your crank, go ahead and have a roast chicken for dinner — and rip the drumstick off to eat with your hands. (No, you don't have to cook. You can always pick up a piping-hot one at the grocery store.) Does it seem to you that a king's chamber might smell of incense? Then find a scent that fits. Your imagination really is the limit!
—There are several websites and computer programs that offer ambient noise, which can enhance your scene.
Ambiance is a particularly good choice for capturing those odd sounds that might make roleplaying spring to life — French street noise, seagulls at the ocean, even warehouse noise can be yours with a few clicks.
—Create an outfit for your role, or shop for outfits together with your partner in advance. Thrift stores can be great resources for this, depending on what you're up to.
—Incense, air freshener, perfume, cologne, flowers, or scented candles can make a huge difference. Scent is one of the strongest memory provokers humans have, and they're not too shabby at invoking fantasies, either. (Just be sure to check with your partner; many people have allergies or sensitivities to artificial scents or types of flowers.)
—Think about the voices and word choices of the people you and your partner(s) will be exploring. Will you be a totally bitchin' surfer dude who says “right on” to sex on the beach? A perfectly poised librarian who may expound upon grammar and vocabulary even whilst your paramour disrobes? A soft-spoken young cowboy just tryin’ to be respectful, ma’am?
—Lighting can make a big difference. Even a small disco ball can bring the groovy '70s right back to your living room. Harsh spotlight-style lights can invoke trysts outside army strongholds. A little dim track lighting and a loveseat might even conceivably be turned into a row of airplane seats during a red-eye flight...
When I first started roleplaying, meeting my partner “in character” was incredibly awkward. We would often laugh through the first ten minutes, feeling very silly. Eventually, I figured out that strong demarcations help a lot. Decide together that when your partner walks into the bedroom, they will be a Na'vi from Avatar, or that once you lay eyes on each other at the coffeeshop, you're the spy. Decide when it will end, too. It may be unglamorous, but usually once a roleplay scene has gone, er, nicely, one of you will have to pee, which usually separates you. It's a useful demarcation.
Once it's over, be sure to debrief. Talk about what was awesome, what was awkward, and what was awful. It'll be the most delightful schooling you've ever had!
Hear hear! Lots of great advice here.