Most people have sexual fantasies. Role-playing involves a certain kind of fantasy and a certain relationship to that fantasy. It requires a conscious acknowledgment of the fantasy; furthermore, it involves sharing that fantasy with a partner, who, presumably, consents to participate in it.
The simplest kind of role-playing involves a person pretending to be a different kind of personality than is typical for him or her (or accessing personal characteristics he/she believes are there, but typically unexpressed). A meek person may pretend to be demanding; a voracious person may pretend to be inhibited. Couples who do this may not even think of it as role-playing, but simply as one of the many playful things they do together in bed.
Somewhat more complicated is role-playing that involves specific roles or even scripts: doctor/patient, pirate/slavegirl, queen/foreign prince, Madeleine Albright/Henry Kissinger. Couples can simply imagine themselves in these roles and speak a sentence or two about it ("You haven't had a checkup in two years. I better examine your prostate"). Or they can get more involved, speaking in role for the majority of the sexual encounter. A few simple props such as an apron, nurse's cap or artist's charcoal can make these games even more involving.
Not surprisingly, some couples take their role-playing out of the bedroom and into the wide world. They don't have genital sex in public, but their behavior is erotically compelling to them. They may do it in an unobtrusive way, calling no attention to themselves and simply enjoying the erotic ambiance they privately create together. Or they may involve the world in their game, whether by costume (an adult woman wearing a cheerleader's skirt), conversation (letting others overhear them talking in role), or behavior (flashing a naked butt at a gas station attendant).
One form of role-playing involves consciously playing with power dynamics. People variously refer to this as bondage, S/M, and discipline; the expression "erotic power-play" covers a wide range of activities and attitudes. Erotic power-play can be mild and barely noticeable; rather extreme, with complicated equipment and well-practiced behaviors; and anything in between. Some people merely dabble with this stuff, while others never have sex without it. There is an enormous S/M subculture, complete with vocabulary, magazines, websites, retailers and conventions. As with non-power-play eroticism, people may choose a predictable script, or they can enjoy improvising, following or leading their partner. Either way, erotic power-play requires trust. In fact, the centrality of trust to the erotic power-play experience is what many aficionados find so thrilling.
Those engaged in erotic role-play face two challenges. First, they have to believe that they are eligible to step outside the usual limits of their everyday personality. Role-playing requires that people either believe that they don't look foolish, or that they don't care if they do. They have to transcend the idea that certain words, behaviors or attitudes belong only to people who are "sexy" (i.e., either young and beautiful, or rich). Similarly, couples have to believe that they can be the "kind of couple" that can do or say whatever they might role-play. Couples have to trust each other's sense of generous engagement and lack of judgment. In these respects, erotic role-playing requires certain psychological and relationship tools that many people lack. Developing those tools can be a transforming experience.
The second challenge involves reentering "real life" after role-playing. People have to look in the mirror, and they have to look into each other's eyes. If they feel embarrassed, ashamed, self-critical or exploited, it will undermine their self-esteem and their relationship. The couple who can look at each other after playing 'apprentice/mentor's wife' and agree that "we can do anything we want" or "aren't we wonderful" or "whatever reinforces our intimacy or pleasure is a good thing" will not only enjoy their erotic role-play - they have an important tool for keeping their relationship desirable.
The difficulty of these two challenges leads some people to do their erotic role-playing with a person other than their regular partner. Whether in an extramarital affair or with a commercial sex worker, some people feel much less exposed or judged in these contexts. Some will feel that their sexuality is more validated in affairs than at home; others will feel less self-conscious acting in ways outside their usual character with hired help.
Regardless of the content or context of the role-playing, it contains no predictions about how people really wish to live; in fact, quite the contrary is often true. Role-playing is a safe arena in which to live other lives - lives that have attractive components, but lie outside a person's destiny. On the other hand, some people do use role-playing to investigate or rehearse possible new behaviors.
Ultimately, erotic role-playing is a way to celebrate two of our most divine gifts: imagination and sexuality.